The general opinion of naturalists is that species, when crossed, are inherently sterile, in order to prevent the fusion of all organic forms with each other. At first there is certainly great probability for this opinion; for species living together in the same region could hardly survive separately if free interbreeding were possible. The importance of the fact that hybrids are very generally sterile has, I think, been greatly underestimated by some recent writers. According to the theory of natural selection, the case is all the more of special importance, as the sterility of the hybrids is not probably advantageous to them, and therefore not more useful by the continued preservation of successive ones[p. 274]Degrees of sterility can be acquired [30] . However, I hope to show that infertility is not a specially acquired or innate characteristic, but is related to other acquired differences.
In treating this subject two classes of facts, fundamentally very different, have usually been confounded, viz., the sterility of two species when they are first crossed, and the sterility of the hybrids obtained from them.
Pure species regularly have reproductive organs of perfect condition, but when crossed together produce few or no offspring. Hybrids, on the other hand, have reproductive organs which are incapable of service, as may be seen from the condition of the male element in plants and animals, while the organs themselves are perfect in structure, as microscopic examination shows. In the first case the two kinds of sexual elements which are to furnish the embryo are complete; in the other they are either not developed at all or only very imperfectly. This distinction is essential if the cause of the sterility occurring in either case is to be considered. The difference has probably been overlooked
The fertility of the varieties, or of those forms which are descended from, or are thought to be, descended from a common parent when they are crossed, and also that of their hybrids[p. 275]is, on my theory, of equal importance with the sterility of species among themselves; for a clear and wide distinction between species and varieties seems to result.
First: The sterility of crossed species and their hybrids. It is impossible to read through the various works and treatises of the two conscientious and admirable observers KÖLREUTER and GÄRTNER , who have devoted almost their whole lives to this subject, without getting a deep impression of the generality of a greater or lesser degree of sterility in crossed species record. K ÖLREUTER makes it a general rule; but he cuts the knot by declaring in ten cases, in which two forms almost universally valid for different species are quite fertile with one another, that they are mere varieties. Also GARDENERSmakes the rule general and denies the ten cases of complete fertility at K ÖLREUTER . But G ARTNER isin this, as in many other cases, it is necessary to carefully count the seeds obtained, to show that there is some diminution in fertility. He always compares the highest number of seeds obtained by two crossed species or their hybrids with their average number in the two pure parent species in their natural state. Yet there seems to me to be a cause of serious error. A plant whose sterility is to be proved must be castrated and, often more important, shut up lest pollen from other plants be carried by insects. Almost all plants belonging to GÄRTNER 'SExperiments served, were planted in pots and, it seems, housed in a room of his house. But that such a procedure must have often impaired the fertility of the plants cannot be denied. For GARTNER himself lists about twenty cases in his table where he castrated the plants and then artificially fertilized them with their own pollen; but the legumes and other such cases where manipulation is more recognized[p. 276]Maassen is difficult, quite aside, half of those twenty plants showed more or less diminished fertility. Since now, moreover , GARTNER crossed the Primula officinalis and Pr. elatior, which we have some reason to consider only as varieties of one species, for a few years in a row, and yet only received fertile seeds once or twice — since he was Anagallis arvensis and A. coerulea, which the best botanists regard only as varieties, found to be quite sterile with one another, and in several analogous cases reached the same result: it seems to me that I can doubt whether many other species are really so sterile when crossed as G ARTNER claims.
On the one hand it is certain that the sterility of some species when crossed is so unequal in degree, and presents in such varied degrees; and, on the other hand, that the fertility of true species is so easily affected by various circumstances, that it is difficult for most practical purposes to estimate say where perfect fertility ends and where sterility begins? I believe that no better proof can be asked for than that the two most experienced observers that ever existed, namely K ÖLREUTER and GARTNER, have come to diametrically opposite conclusions about the same species. It is also very instructive to compare the arguments put forward by our best botanists as to whether this or that doubtful form should be considered a species or a variety, with that from fertility or sterility as reported by various hybrid breeders or the Evidence taken from experiments carried out by the authors themselves over several years. It may be shown from this that neither fertility nor sterility furnishes a clear distinction between species and varieties, the evidence based on it gradually disappearing, and consequently remaining doubtful like the other evidences taken from organic structure and action.
With regard to the infertility of hybrids through inbreeding, GARTNER has made a number of experiments[p. 277]and protected the inbreeding for 6-7, and in one case even 10 generations from any cross-breeding with either of the two progenitors, but expressly asserted that their fertility had never increased, but rather greatly decreased. I have no doubt that this is usually the case, and that fertility often falls suddenly in the first generations. Notwithstanding this, I believe that in all these attempts the fertility has been diminished by an independent cause, viz., by excessive inbreeding. I have collected a great quantity of facts, showing that excessive inbreeding diminishes fertility, while, on the other hand, cross-breeding with another individual or variety increases fertility, therefore I cannot doubt the correctness of this opinion, which is almost universally spread among breeders. Hybrids are seldom raised in large numbers for experimentation, and as the older species or other closely allied species usually grow in the same garden, the visits of insects during the time of flowering must be carefully discouraged, hence hybrids for each generation usually by their own pollen must be fertilized; and I am convinced that this affects their fertility, which is already weakened by their hybrid nature. This conviction is reinforced by one of G and as the parent species, or other closely allied species, usually grow in the same garden, the visits of insects during the time of flowering must be carefully avoided, hence hybrids for each generation must usually be fertilized by their own pollen; and I am convinced that this affects their fertility, which is already weakened by their hybrid nature. This conviction is reinforced by one of G and as the parent species, or other closely allied species, usually grow in the same garden, the visits of insects during the time of flowering must be carefully avoided, hence hybrids for each generation must usually be fertilized by their own pollen; and I am convinced that this affects their fertility, which is already weakened by their hybrid nature. This conviction is reinforced by one of GARTNER repeatedly asserted that the less fertile hybrids, even if they were artificially inseminated with hybrid pollen of the same kind, despite the often poor success of the treatment, sometimes decidedly continue to increase in fertility. Now, in artificial fertilization, the pollen is often accidentally taken (as I know from my own experiments) from the anthers of a flower other than that to be fertilized, so that a crossing is thereby effected between two flowers, but usually of the same plant. Furthermore, if such a careful observer as GARTNER has castrated his hybrids in the course of his compound experiments, this would result in a crossing with the pollen in each generation[p. 278]another flower either from the same plant or from another plant of the same hybrid condition. And so the strange phenomenon of increased fertility in successive generations of artificially inseminated hybrids can, I believe, be explained by the avoidance of too close inbreeding.
Let us now turn to the results obtained by the experiments of the third of the most experienced breeders of hybrids, the Honorable and Reverend W. H ERBERT . He also expressly assures that some hybrids are perfectly fertile and no less breedable than each of the parent species in itself, just as KÖLREUTER and GÄRTNER declare a certain degree of sterility when different species are crossed with each other as a general law of nature. His experiments referred to some of the same species which had also been used in GARNER 'S experiments . The difference in the conclusions reached by both may, I believe, be partly deduced from HERBERT 'S great experience in flower- growing , and in part from having hothouses at his disposal. Of his many important results, I would like to emphasize just one example here, namely that "each oak tree fertilized with Crinum revolutum on a stick of Crinum capense also produced a plant, which I (he says) have never noticed with natural fertilization." We we therefore have here the case of perfect and even more than perfect fertility in the crossing of two different species.
This case with Crinum leads me to a peculiar fact, namely, that in some species of Lobelia, and several other genera, there are individual plants which are much more readily fertilized with the pollen of a distinct species other than their own; and it seems to be the same with all individuals of almost all species of Hippeastrum. For it has been found that these plants, fertilized with the pollen of another species, set seed, but are quite sterile with their own pollen, though the same[p. 279]perfectly good and is able to fertilize other species. Thus certain single plants, and all individuals of certain species, can serve much more readily for the breeding of hybrids than can be self-fertilized. A bulb of Hippeastrum aulicum z. B. brought four flowers; three of these were fertilized with their own pollen, and the fourth then provided with the pollen of a hybrid bred from three other distinct species, and the result was that "the ovaries of the first three flowers soon ceased to grow, and after a few days perished altogether, while the ovary of the flower provided with the hybrid pollen rapidly increased and matured, yielding good seeds which thrived vigorously”. In 1839 H ERBERT wrote to methat he had continued the experiment for five years and every year with the same success. Other observers had the same success with Hippeastrum and its sub-species, as well as with some of the other genera, viz., Lobelia, Passiflora, and Verbascum. Although these plants appeared perfectly healthy on experiment, and both acorns and pollen of one and the same flower proved perfectly fine when fertilized by other species, yet they were functionally insufficient for mutual self-fertilization, and we must therefore conclude that the plants were in an unnatural condition. At any rate, these phenomena show on what petty and mysterious causes the greater or lesser fertility of species in cross-breeding, as opposed to self-fertilization, sometimes depends.
The practical experiments of gardeners, though not carried out with scientific accuracy, also deserve some attention. It is well known in what a complicated manner the species of Pelargonium, Fuchsia, Calceolaria, Petunia, Rhododendron, etc. have been crossed, and yet many of these hybrids plant seeds. Thus Herbert asserts that a hybrid of Calceolaria integrifolia and C. plumbaginea, two species very dissimilar in their general constitution, "rejuvenated itself from seed as perfectly as if it had belonged to a natural species from the mountains of Chile". I[p. 280]I have taken some trouble to learn the reason for the fertility of some rhododendrons obtained by cross-crossing, and have obtained the certainty that several of them are perfectly fertile. Mr. C. N OBLE z. B. tells me that he raises canes of a hybrid of Rhododendron ponticum and Rh. With proper treatment, the fertility of the hybrids would decrease in successive generations in the same way as GARTNERassured, this fact should be known to our plantation owners. Garden-friends raise big beds full of the same hybrids; and these alone enjoy right treatment; for here alone can the distinct individuals of the same hybrid form interbreed by the action of insects, and escape the injurious influences of close inbreeding. Of the effect of insect activity, any one can convince himself by examining the flowers of the more sterile Rhododendron forms, which do not produce pollen; for he will find their scars all covered with seed-dust carried from other flowers.
As to animals, much fewer exact experiments have been made on them. If our systematic arrangements are worthy of trust, i. H. if the genera of animals are as distinct from one another as are those of plants, then we may assert that animals farther apart on the scale of nature can be interbred than is the case with plants; on the other hand, the hybrids seem to be more sterile. I doubt whether even a statement from a quite fertile animal hybrid can be regarded as fully authenticated. It must be remembered, however, that few animals reproduce profusely in captivity, and therefore few real experiments can be made with them. So you have z. B. crossed the canary with nine other species of finches,[p. 281] so we have no right to expect that the first hybrids of them and the canary should be perfectly fertile. Likewise, as far as the fertility of the comparatively fertile hybrids is concerned in later generations, I hardly know of an example of two families of the same hybrid being brought up at the same time by different parents in order to be able to avoid the bad consequences of excessive inbreeding; on the contrary, one has in every succeeding generation, ignoring the constantly repeated admonitions of all breeders, usually brothers and sisters mated together. And so it is not at all surprising that the inherited sterility of the hybrids increased with each generation. If we always mated brothers and sisters of pure species with the intention of working towards this,
Though I know of no well-authenticated case of perfectly fertile animal hybrids, I have some reason to believe that the hybrids of Cervulus vaginalis and C. Reevesi, and those of Phasianus Colchicus and Ph. torquatus, are perfectly fertile. In particular, there is no doubt that these two closely allied species of pheasants, both as Ph. versicolor from Japan , interbreed in the woods of some parts of England , and furnish offspring. After the recent ones in FranceFrom experiments carried out on a large scale, it seems as if two species so different from each other, such as the hare and rabbit, are able, if they can be induced to mate with one another, to furnish a mostly quite fertile offspring. The hybrids of the common goose and the swan goose (Anser cygnoides), two species so different that they are usually placed in different taxa, have often produced offspring in this country with one of the pure parent species, and in one case even among themselves. This has been effected by Mr. E YTON , who reared two hybrids of the same parents but different breeds, and then of both[p. 282]together received no less than eight offspring from one nest. In India , on the other hand, the geese obtained by crossbreeding must be far more fertile, as two excellent judges, viz. Mr. BLYTH and Capt. H UTTON , have assured me that flocks of these bastard goose are kept there in various parts of the country; and as this is done for the sake of utility where the pure parent species do not exist at all, they must necessarily be very fertile.
Recent naturalists have largely accepted a teaching emanating from PALLAS , namely that most of our domestic animals are descended from two or more wild species, which have since interbred through interbreeding. According to this, either the parent species must have produced very fertile hybrids right at the beginning, or the hybrids must have become fully fertile only in later generations in a tame state. This last alternative seems more likely to me. I take e.g. Suppose, for example, that our dogs are descended from several wild species, and yet, excepting perhaps certain ones, are in South Americakept domestic dogs all perfectly fertile with each other; but the analogy raises great doubts in my mind that the various parent species of the same species should at first have mated with each other voluntarily and at once have produced quite fertile hybrids. I previously drew attention to the East Indian bull ox [or zebu?]. If I now describe his way of life, his external physique and his osteological peculiarities (as Prof. R ÜTIMEYER has clearly demonstrated) with our EnglishCompare races, it is as distinct a species as any in the world; and yet I have lately received proof that the offspring obtained by crossing the two is mutually fertile. With this view of the origin of many of our domestic animals, we must either give up the belief in the almost universal sterility of mating different animal species with one another, or else regard sterility not as an indestructible consequence of such a cross, but as a result that can be removed by taming .
[p. 283]
Finally, reviewing all the facts established concerning the crossing of plant and animal species, we come to the conclusion that a certain degree of sterility in the first cross and the hybrids resulting therefrom, though an extremely common phenomenon, is as of the present state our knowledge should not be regarded as necessarily general.
Laws governing the sterility of first crosses and hybrids. ) Let us now consider a little more closely the circumstances and the rules which determine the comparative sterility of the first cross and the hybrids. Our main task will be to know whether, according to these rules, sterility of the species with one another turns out to be an inherent quality, the purpose of which would be to prevent the crossing of species to the extreme fusion of forms, or whether this turns out not to be the case. The following rules and conclusions are taken mainly from GÄRTNER 'S admirable works on hybrid production in plants [31]. I have taken much pains to learn how far these rules apply to animals, and though our experience of hybrid animals is very scanty, I have been amazed to see to what extent the same rules apply to both kingdoms are valid.
It has already been remarked that the fertility of both the first cross and the resulting hybrids is graded from zero to perfection. It is amazing how many peculiar ways this gradation can be shown; but only the barest outline of the facts can be given here. If the pollen one[p. 284]If a plant from one family is placed on the stigma of a plant from another family, it has no more effect than an equal amount of inorganic dust. But if one brings seed dust from species of a phylum onto the stigma of a species of the same phylum, the result becomes more favorable, but again so unequal among different species that, by means of the number of seeds produced each time, all gradations from that zero up to complete fertility, and, as we have seen, in some abnormal cases even beyond what is usual in self-fertilization. Thus among the hybrids themselves there are some which have never produced, nor are likely to ever produce, even a fertile seed, even with the pollen from either of the two pure parent species. But in some of these cases a first trace of the action of such pollen has been shown, in causing an earlier withering of the flower of the hybrid plant to which it had been brought; and rapid wilting of a flower is known to be a sign of incipient fertilization. At this extreme degree of sterility, hybrids follow, which through self-fertilization produce an increasing number of seeds until they are completely fertile.
Hybrids obtained from such two species, which are very difficult to cross, and seldom produce an offspring, tend to be very sterile themselves. But the parallelism between the difficulty of effecting a first cross, and that of fertilizing a resulting hybrid—two very commonly confounded classes of facts—is by no means rigorous. For there are many cases where two pure species may be mated with unusual ease, and may furnish numerous hybrids, but which are exceedingly sterile. On the other hand, there are species which are seldom or extremely difficult to cross, but their hybrids, once existing, are very prolific. And these two cases, so opposed, may occur within the same clan, as e.g. B.
[p. 285]
The fertility of both the first crosses and the hybrids is more easily endangered than that of the pure species by unfavorable conditions. But the degree of fertility is equally variable in itself; for the result is not always the same in crossing the same two species under the same circumstances, but depends in part on the condition of the two individuals chosen at random for the experiment. So it is with the hybrids, the degree of fertility often showing quite different in different individuals raised from seeds of a single capsule and exposed to the same conditions.
Expressing systematic affinitythe similarity of different species in organic structure and activity is to be indicated, especially in those parts which are of great physiological importance and differ only slightly from one another in related species. Now the fertility of the first cross between two species and the resulting hybrids depend to a large extent on this "systematic relationship". This is clearly evident from the fact that hybrids have never been obtained from two species, which the systematists place in different families, while on the other hand it is usually easy to pair closely related species with one another. But the relationship between systematic relationship and ease of crossing is by no means a strict one. For a multitude of cases of very closely related species could be cited, which cannot be made to mate at all or only with great difficulty, while sometimes very different species can be crossed with great ease. In the same family two genera may exist together, one of which, like Dianthus, contains many species which are very easy to cross, while those of the other, e.g. B. Silene, resist the most persistent attempts at cross-breeding to such an extent that it has not yet been possible to obtain a hybrid between the most closely allied species of the same species. Yes, even within the limits of one and the same clan, one appears of which one, like Dianthus, contains many species which are very easy to cross, while those of the other, e.g. B. Silene, resist the most persistent attempts at cross-breeding to such an extent that it has not yet been possible to obtain a hybrid between the most closely allied species of the same species. Yes, even within the limits of one and the same clan, one appears of which one, like Dianthus, contains many species which are very easy to cross, while those of the other, e.g. B. Silene, resist the most persistent attempts at cross-breeding to such an extent that it has not yet been possible to obtain a hybrid between the most closely allied species of the same species. Yes, even within the limits of one and the same clan, one appears[p. 286]such difference. So are e.g. For example, the numerous Nicotiana species have been crossed with one another more than most of the other genera, and GARTNER has found that N. acuminata, which is by no means a particularly different species, persistently resisted all attempts at fertilization, so that out of eight other species of Nicotiana neither could fertilize it nor be fertilized by it. And analogous facts could still be quoted many more.
No one has yet been able to ascertain what kind or degree of difference in any recognizable character is sufficient to prevent the interbreeding of two species. It can be shown that plants which differ most widely in habit and general dress, which show very sharp differences in all parts of their flowers, even to the pollen, or in the fruit, or in the cotyledons, may be crossed together. Annual and perennial species, winter bare and evergreen trees, plants made for the most varied locations and the most opposed climates can often be easily crossed with each other.
Under mutual crossingI understand the case in two ways, where e.g. B. mating a horse stallion to a donkey and then a donkey stallion to a horse mare; one may then say that these two species have been mutually crossed. The greatest possible difference often occurs in the ease of mutual crossing. Such cases are most important, as showing that the susceptibility to cross-breeding between any two species is often quite independent of their systematic affinity, or of any discernible difference in their whole organization. On the other hand, these cases also show clearly that that susceptibility is related to differences in the constitution of the body, which are imperceptible to us and are limited to the reproductive system.OILREUTER observed[p. 287]been. Thus, for example, Mirabilis jalapa can be easily fertilized by the pollen of M. longiflora, and the resulting hybrids are sufficiently fertile; but more than two hundred times over the course of eight years, K ÖLREUTER tried in vain to fertilize M. longiflora with pollen from M. Jalapa. And so a few other examples could be given. T HURET has made the same remark on some sea plants, and G ARTNER yet moreover found that this phenomenon is exceedingly common in a lesser degree. He has observed them himself among forms which many botanists consider only varieties of a single species, such as Matthiola annua and M. glabra. It is also a remarkable fact that the hybrids of the two mutually crossed, though originating from the same two parent species, differ in their fertility, usually in a slight, but sometimes in a great degree.
Many other peculiar rules from G ARTNERextract, such as B. that some species are altogether very easily used for cross-breeding with others, while other species of the same genus have the inherent power of impressing on the hybrids a close resemblance to them; but the two faculties are not necessarily related to each other. There are hybrids which, instead of being as usual intermediate between their two parent species, always closely resemble only one of them; and it is precisely these hybrids, so externally similar to the parent species, that are, with rare exceptions, extremely sterile. On the other hand, among those hybrids which are accustomed to occupying an intermediate position between their elders, abnormal individuals sometimes occur which extremely resemble one of the pure progenitors; and these bastards are then usually extremely sterile, although the intermediate forms that have sprung with them from the same fruit capsule tend to be very fertile. These phenomena show how completely independent the fertility of the hybrids is from the degree of their resemblance to their two progenitors.
[p. 288]
From the rules hitherto given concerning the fertility of the first crosses and the hybrids thereby obtained, it follows that when forms which must be regarded as good and distinct species are mated with one another, their fertility is in all degrees from zero to even may vary beyond the degree of perfect fertility that occurs under ordinary conditions. Further, their fertility is not only extremely sensitive to favorable and unfavorable conditions, but is inherently variable. The fertility is not always the same in terms of strength with the first crossing and the resulting hybrids. The fertility of the latter bears no relation to their external resemblance to their two elders. The ease of a first cross between two species does not depend on their systematic affinity nor on their resemblance to each other. This last result is chiefly demonstrable from the difference in the result of cross-crossings of two same species, where mating is usually somewhat, but sometimes much easier or more difficult, according as the father is taken from one or the other of the two species crossed. Finally, the two kinds of hybrids obtained by cross-crossing often differ in their fertility. sometimes, however, much easier or more difficult, depending on whether the father is taken from one or the other of the two crossed species. Finally, the two kinds of hybrids obtained by cross-crossing often differ in their fertility. sometimes, however, much easier or more difficult, depending on whether the father is taken from one or the other of the two crossed species. Finally, the two kinds of hybrids obtained by cross-crossing often differ in their fertility.
Now the question arises whether it follows from these peculiarly complicated rules that the comparative sterility of species when they are crossed has the purpose of preventing their interbreeding in the state of nature! I do not think so. For why would the degree of infertility vary so extraordinarily in this case, since we must assume that contraception is equally important for everyone? Why would there even be an innate difference between individuals of the same species? To what end should some species be so easy to cross, and yet produce very sterile hybrids, while others are very difficult to mate, and produce perfectly fertile hybrids? What is the use of the fact that the two products of an intercrossing between the same species often behave so very differently?[p. 289]What purpose, one might even ask, should the possibility of delivering bastards serve at all? It seems a curious arrangement, however, that species have the power of producing hybrids, but whose further reproduction is checked by various degrees of sterility, unrelated to the facility of the first interbreeding of two parents of different species.
On the other hand, the foregoing rules and facts seem to me to prove plainly, that the sterility of both the first crosses and the hybrids depends chiefly on unknown conditions in the reproductive systems of the crossed species. The differences are of such a peculiar and limited nature, that in mutual crossings between two species the male element of one is often of luxuriant effect on the female of the other, while in crossing in the other direction the opposite occurs. It will be fair to set out a little more fully, by example, what I mean by the remark that sterility is connected with other causes, and does not constitute a special peculiarity. The ability of a plant to branch or not to branch and bud onto another is so completely irrelevant to its growth in the natural state that nobody will consider this ability to be a special arrangement of nature, but everyone will be inclined to assume it coincide with differences in the laws of growth of the two plants. The reason why one species does not want to take hold of the other can sometimes be found in a different mode of growth, hardness of the wood, nature of the sap, time of flowering, and the like; in very many cases, however, no cause can be found. For even very significant differences in the size of the two plants, or in woody and herbaceous, evergreen and deciduous nature, and even their adaptation to quite different climates, do not always constitute an obstacle to their grafting together. As with hybrid formation, so with grafting, the ability is through systematic[p. 290]affinity limited; for wood species from completely different families have never been successfully placed on top of one another, while on the other hand closely related species of a clan and varieties of a species can usually, but not always, be easily grafted onto one another. But this ability is just as little determined by systematic relationship in an absolute way as is that of hybrid formation. For even if many different taxa of a family have succeeded in grafting one another, yet in other cases even species of the same taxa do not adopt one another. The pear tree can be branched much more easily onto the quince tree, which has been raised to a genus of its own, than onto the apple tree, which belongs to the same family with it. Even different varieties of the pear do not strike the quince tree with equal ease,
Just as, according to GARTNER , there is sometimes an innate difference in the behavior of the individuals of two species to be crossed, SAGARET also believes in an innate difference in the behavior of the individuals of two species to be grafted on to each other. Just as the ease of the two kinds of pairings is often very unequal in alternate crossings, so it is often the case with mutual grafting. So the common gooseberry z. B. be branched on the currant bush, but this will hit hard on the gooseberry bush.
We have seen that the sterility of hybrids, whose reproductive organs are of an imperfect condition, is quite a different matter from the difficulty of mating two pure species with complete organs; but both cases run parallel to one another to a certain extent. Something similar also occurs with plugging; for T HOUIN found that the three species of Robinia, which had produced abundant seed on their own roots, and were easily branched on one another, were rendered sterile by inoculation; while, on the other hand, certain Sorbus species, a[p. 291]the other planted, yielded twice as much fruit as from their own roots. This reminds us of the extraordinary cases above alluded to in Hippeastrum, Lobelia, &c., which fructify much more profusely when provided with pollen of another species than when provided with their own pollen.
We see, therefore, that while there is a clear and thorough distinction between the mere adhesion of grafted canes and the cooperation of male and female progenitors for the purpose of reproduction, there is a certain parallelism between the effects of inoculation and the fertilization of different species with announce each other. If we regard the curious and intricate rules governing the facility of grafting as connected with unknown differences in the vegetative organs, I think we must also consider the much more complex ones governing the facility of the first crosses with unknown differences in their reproductive systems view related. These differences follow, as might be expected, to a certain degree of systematic affinity, by which term every kind of resemblance and dissimilarity between organic beings is meant to be expressed. The facts do not seem to me to indicate in any way that the greater or lesser difficulty in grafting and crossing different species on and with one another is a peculiar peculiarity, though the same is as important in crossing to the permanence and continuity of species-forms as in grafting is essential for their prosperity.
Causes of sterility in first crosses and hybrids. ) Let us now look a little more closely at the probable causes of the sterility of the first crosses and the hybrids. These two cases are fundamentally different, as, as above remarked, the male and female genitals are perfect in the mating of two pure species, but imperfect in hybrids[p. 292]are. Even in first crosses, the greater or lesser difficulty in effecting mating appears to depend on several different causes. It often lies in the physical impossibility for the male element to reach the oak, as is the case with plants whose pistil is so long that the pollen tubes cannot reach down into the ovary. Thus it has also been observed that when the pollen of one species is placed on the stigma of a species only distantly related, the pollen-tubes emerge, but do not penetrate the surface of the stigma. In other cases the masculine element may reach the feminine, but be unable to bring about the development of the embryo, as Thuret's experiments show seems to emerge with seaweed. We can no more explain these facts than why certain species of wood cannot be grafted onto others. Finally, it can also happen that an embryo begins to develop but soon perishes. This last possibility has not been sufficiently elucidated; but I think after Mr. H EWITT'sreceived communications, having had much experience in hybrid breeding of the gallinaceous birds, that the premature death of the embryo is a very frequent cause of the failure of the first crosses. I was very little inclined to believe it at first, because hybrids, once born, tend to be very strong and long-lived, as mules and hinnies show. Moreover, hybrids are in very different conditions before and after birth. Born and living in an area where their two elders also live, the living conditions may well suit them. But a hybrid participates only half in the organic formation and activity of its mother, and thus before birth, so long as it is still in the womb, or in the eggs and seeds produced by the mother,[p. 293]are extraordinarily sensitive to harmful and unnatural living conditions.
With regard to the sterility of hybrids, whose sexual organs are imperfectly developed, the matter is quite different. I have stated several times that I have collected a great deal of evidence, showing that when plants and animals are torn from their natural conditions, it is the reproductive organs which are chiefly affected. This is indeed the great limit to the taming of animals. There are many similarities between the sterility caused by this and that of hybrids. In both cases sterility is independent of general health and is often accompanied by increased size and voluptuousness. In both cases the sterility comes in many degrees; in both the male element suffers the most, but sometimes the female still more than the male. In both fertility goes up to a certain stage in step with systematic relationship; for whole groups of plants and animals are rendered impotent by the same unnatural conditions, and like groups of species tend to produce sterile hybrids. On the other hand, a single species in a group will sometimes withstand great changes in external conditions with undiminished fertility, and certain species in a group will furnish unusually fertile hybrids. No one can predict, until he has tried, whether this or that animal in captivity, or whether this or that foreign plant while being cultivated, will reproduce well. nor whether any two kinds of a clan will produce more or less sterile hybrids together. Lastly, when organic beings are placed, during several generations, in conditions unnatural to them, they are exceedingly apt to vary, which I believe results from their reproductive systems being preferentially affected, though to a lesser degree than when there is utter sterility follows. So it is with bastards;[p. 294]for hybrids are apt to vary greatly in successive generations, as every breeder has experienced.
Thus we see that when organic beings are placed in new and unnatural conditions, and when hybrids are produced by the unnatural crossing of two species, the reproductive system, quite apart from general health, is peculiarly affected by sterility. In the one case the conditions of life have been disturbed, though often to a degree imperceptible to us; in the other, namely, in the hybrids, those relations have remained unchanged, but the organization has been disturbed by the fact that two different structures and constitutions of the body have been mixed up. For it is hardly possible for two organizations to be combined into one, without causing any disturbance in the development, or in the periodic activity, or in the mutual relations of the various parts and organs to one another, or finally in the vital relations. When hybrids are able to reproduce among themselves, they transmit from generation to generation to their offspring the same union of two organizations, and we should not be surprised, therefore, to see their sterility, though subject to some vacillation, seldom diminishing.
We must, however, confess that, barring unfounded hypotheses, we are unable to apprehend certain facts concerning the sterility of hybrids, such as: B. the unequal fertility of the two kinds of hybrids from cross-crossing, or the increasing sterility of those hybrids which happen to be very similar, or exceptionally, to one of their parents. Nor do I pretend to get to the bottom of the matter from the foregoing remarks; for we have no explanation why an organism becomes sterile under unnatural conditions of life. All I have wished to show is, that in two cases similar in some respects sterility is the same result in one case because the outer[p. 295]conditions of life, and in the other because the organization itself has been disturbed by combining two formations into one.
It may seem strange, but I suspect that there is a like parallelism in another, related, but intrinsically very different set of facts. It is an ancient and almost universal belief, based, to my knowledge, on a body of experience, that slight changes in the external conditions of life are beneficial to all living beings. We therefore see farmers and gardeners constantly exchanging their seeds, tubers, etc., transferring them from one soil and climate to another and finally probably back again. During the recovery of animals we often see them taking great advantage of this or that change in their habits. So also with plants and animals there is ample evidence that a cross between very different individuals of a species, namely between those of different tribes or sub-races, giving strength and fertility to the offspring. Indeed, I believe, from the facts adduced in the fourth chapter, that a certain degree of crossbreeding is indispensable even to hermaphrodites, and that close inbreeding between the nearest kin continues for some generations, especially when kept under the same conditions of life, finally delivers weak and sterile offspring.
It seems to me, then, that, on the one hand, slight changes in the conditions of life are advantageous to all organic beings, and, on the other hand, that slight crosses, viz., between distinct stocks and slight varieties of a species, give vigor and strength to the offspring. On the other hand, we have also seen that major changes in conditions, and especially those of a certain kind, can often render the organism to a certain degree sterile, as can major crosses, namely between very different or in certain respects deviating males and males[p. 296]Females produce hybrids which are usually somewhat sterile. I cannot persuade myself that this parallelism is due to mere chance or deception. Both series of facts seem to be linked together by a common but unknown bond, essentially connected with the principle of life.
Fertility of crossed varieties and their hybrids.) It may be objected to us as a very strong argument that there must be some essential difference between species and varieties, and some error running through all the foregoing remarks, since varieties are indeed, however much they differ in their outward appearance , yet easily interbreed and produce perfect fertile offspring. With a few exceptions, which will be demonstrated immediately, I fully admit that Diess mostly behaves invariably like this. But this case still presents great difficulties; for when we consider the varieties occurring in nature we are immediately ensnared in hopeless difficulties, for as soon as two forms hitherto regarded as varieties show themselves reasonably sterile with each other, these are at once elevated to species by most naturalists. So are e.g. B. the red and the blue Anagillis, the light and the dark yellow Cowslip, which most of our best botanists consider to be mere varieties, according to GARTNER are not completely fertile when crossed and are therefore described by him as undoubted species. If we conclude from this in a circle, then the fertility of all naturally occurring varieties must be regarded as proven.
Even when we turn to the proven or conjectured varieties produced in the culture state, we still find ourselves involved in doubt. Because if it z. For example, it is certain that the German Spitz mates with the fox more easily than other breeds of dog, or that certain domestic dogs native to South America do not really mate with European ones[p. 297]Crossbreeding dogs, the explanation that will occur to everyone and is probably the correct one, is that these dogs are descended from different wild species. Notwithstanding this, the perfect fertility of so many cultivated varieties, so widely different in their outward appearance as those of the pigeon and the cabbage, is a remarkable fact, especially when we consider how numerous species there are which outwardly very much resemble each other similar, but quite sterile when crossed with each other. Various considerations, however, make the fertility of the cultivated varieties seem less strange than it initially is. Because first, we must remember how little we know of the true cause of sterility, both of species crossed and alienated from their natural conditions of life. On this last point, space has not permitted me to enumerate the many curious facts which I have gathered; as to sterility, it is reflected in the difference between the two hybrids of cross-crossing, and in the peculiar cases in which a plant is more easily fertilized by foreign pollen than by its own pollen. When we reflect upon this and other cases, such as that to be reported later, of the differently colored varieties of Verbascum thapsus, we must feel how great is our ignorance and how small is the probability for us to understand whence it comes that when crossed certain forms are fertile and others sterile. Secondly, it can be clearly shown that the mere outward dissimilarity between two species does not necessarily mean that they are more or less sterile when crossed; and the same rule will apply to the cultivated varieties. Thirdly, some eminent naturalists believe that a long-lasting state of domestication or culture is apt to obliterate more and more in successive generations the sterility of hybrids, which at first have been only slightly sterile; and if this is the case, that the mere outward dissimilarity between two species does not necessitate their greater or lesser sterility in the case of a cross; and the same rule will apply to the cultivated varieties. Thirdly, some eminent naturalists believe that a long-lasting state of domestication or culture is apt to obliterate more and more in successive generations the sterility of hybrids, which at first have been only slightly sterile; and if this is the case, that the mere outward dissimilarity between two species does not necessitate their greater or lesser sterility in the case of a cross; and the same rule will apply to the cultivated varieties. Thirdly, some eminent naturalists believe that a long-lasting state of domestication or culture is apt to obliterate more and more in successive generations the sterility of hybrids, which at first have been only slightly sterile; and if this is the case, to blur more and more in successive generations; and if this is the case, to blur more and more in successive generations; and if this is the case,[p. 298]thus we shall certainly not expect to see sterility appear and disappear under the influence of nearly the same conditions of life. Finally, and this seems to me to be by far the most important consideration, man brings forth new plant and animal races in a state of culture by the power of planned or unconscious selection for his own benefit and pleasure; he will not and cannot make the small differences in the reproductive system, or other differences correlated with the reproductive system, the object of his breeding. The products of culture and domestication are far less perfectly adapted to the climate and other physical conditions of life than are those of nature; for they can usually be transplanted to other regions of different character without harm. Man feeds these various variations with the same food, treats them almost in the same way, and will not change their general habits. Nature acts uniformly and slowly, during immeasurable periods of time, upon the whole organization of creatures in a manner for their own good; and so it may directly, or more probably indirectly, by correlation, also modify the reproductive system in the various descendants of the same species. If you take into account this difference in the breeding process on the part of man and nature, it will no longer be surprising that there is some difference in the results as well. treats them almost the same way and doesn't want to change their general way of life. Nature acts uniformly and slowly, during immeasurable periods of time, upon the whole organization of creatures in a manner for their own good; and so it may directly, or more probably indirectly, by correlation, also modify the reproductive system in the various descendants of the same species. If you take into account this difference in the breeding process on the part of man and nature, it will no longer be surprising that there is some difference in the results as well. treats them almost the same way and doesn't want to change their general way of life. Nature acts uniformly and slowly, during immeasurable periods of time, upon the whole organization of creatures in a manner for their own good; and so it may directly, or more probably indirectly, by correlation, also modify the reproductive system in the various descendants of the same species. If you take into account this difference in the breeding process on the part of man and nature, it will no longer be surprising that there is some difference in the results as well. and so it may directly, or more probably indirectly, by correlation, also modify the reproductive system in the various descendants of the same species. If you take into account this difference in the breeding process on the part of man and nature, it will no longer be surprising that there is some difference in the results as well. and so it may directly, or more probably indirectly, by correlation, also modify the reproductive system in the various descendants of the same species. If you take into account this difference in the breeding process on the part of man and nature, it will no longer be surprising that there is some difference in the results as well.
I have hitherto spoken as if the varieties of the same species, when crossed, were for the most part invariably fertile. But it seems to me impossible to ignore the evidence of the existence of a certain measure of sterility in a few cases, of which I shall lately relate. The evidence is at least as good as that which leads us to believe in the sterility of a multitude of species [in crossing?], and is borrowed from opposing witnesses who in all other cases fertility[p. 299]and infertility as good species criteria. GARTNER kept for some years a variety of dwarf corn with yellow seeds and a large variety with red seeds, which grew close together in his garden; and although these plants are of separate sexes, yet they never naturally interbreed. He then fertilized thirteen spikes of flowers [32] of one with the pollen of the other; but only one stick gave some seeds, and only five grains.
The mode of treatment in this case cannot have been injurious, the plants having separate sexes. No one, to my knowledge, has considered these two varieties of corn to be different species; and it is essential to note that the mongrels raised from them were completely fertile, so that G ARTNER himself did not dare to declare those varieties to be two different species.
GIREAU DE B UZAREINGUES crossed together three varieties of cucumbers, which, like corn, are of separate sexes, and affirmed that the greater their difference, the more difficult their cross-fertilization. How far this experiment deserves trust I do not know, but the three forms used for the same have been established as varieties by Sagaret , who bases his distinction of species principally on their sterility.
Far stranger and at first almost unbelievable is the following case; however, it is the result of many years of continuous experiments on nine Verbascum species, which are all the more important here because they come from GARTNER'N , who is just as excellent an observer as a decided opponent of the opinion that the yellow and white varieties of the same Verbascum species, when crossed, yield fewer seeds than each yields when fertilized with pollen from flowers of their own color. He now explains that when yellow and white varieties of a species with yellow and white[p. 300]varieties of another species are crossed, more seeds are obtained by mating the like-colored varieties than by mating the dissimilar varieties. And yet between these varieties of Verbascum there is no other difference than in the color of their flowers, and one color sometimes arises from seeds of the other colored variety.
From experiments I have made on certain varieties of the Rose Mallow, I should like to suppose that they present similar phenomena.
KÖLREUTER , whose accuracy has been confirmed by every subsequent observer, has proved the remarkable fact that a variety of tobacco, when crossed with an entirely different species distant from it, is more fertile than with varieties of the same species. He made Experiments with five forms, which are generally valid for varieties, which he also proved by the strictest test, namely by cross-crossings, which provided nothing but completely fertile mongrels. But one of these five varieties, whether it came into play as father or mother, always gave less sterile hybrids when crossed with Nicotiana glutinosa than the other four varieties. The reproductive system of this one variety must therefore have been in some way less modified.
With great difficulty in confirming the sterility of varieties in the natural state, because every variety that is somewhat sterile when crossed would soon be generally declared to be a species, as well as from the fact that man in his artificial selections only looks at the outer ones characters and does not intend to bring about hidden and functional differences in the reproductive system, I believe I am justified in concluding from the compilation of all the facts that the fertility of varieties among one another is by no means a general rule and therefore also not suitable as a basis for distinguishing between varieties and species available. The fertility that usually takes place among the varieties among one another appears[p. 301]not sufficient for me, with our utter ignorance of the causes of both fertility and sterility, to overturn my view of the very general but not constant sterility of first crosses and hybrids, namely that it does not represent a special quality in itself, but with other slowly developed modifications, especially in the reproductive systems of the interbred forms.
Hybrids and mongrels compared regardless of their fertility. ) The offspring of the crossed species and that of the varieties can be compared with one another in several respects, independently of the question of fertility. GARDENERS _, whose persistent desire has been to draw a sharp line of distinction between species and varieties, has been able to discover very few, and apparently very insignificant, differences between the so-called hybrids of species and the hybrids of varieties, while they are found in many others essential relationships completely the same. I can only briefly discuss this subject here. The most important difference that has emerged is that in the first generation, hybrids are more variable than hybrids; but GARTNER admits that hybrids of species that have been cultivated for a long time are often very variable even in the first generation, and I myself have very good evidence for this fact. GARDENERS _further admits that hybrids between very closely related species are more variable than those of widely separated ones; and it follows that the difference sought in degree of variability gradually diminishes. When mongrels or more fertile hybrids are propagated in themselves for a few generations, it is recognized that the variability of their offspring increases to an extraordinary degree; on the other hand, a few cases can be given where hybrids, as well as mongrels, have long maintained their monotonous character. But the variability is in the successive[p. 302]Generations of mongrels perhaps greater than hybrids.
This greater variability of the mongrels over the hybrids does not strike me as surprising at all. For the parents of mongrels are varieties, and are mostly tame and cultivated varieties (as very few experiments have been made with wild varieties), and as a rule to suppose that their variability is still a recent one, it is therefore to be expected that the same often still endure and amplify the variability already arising from the crossing. The lesser degree of variability in first-crossed or first-generation hybrids, in contrast to their extreme variability in later generations, is a peculiar and noteworthy fact; for it leads to the view I have formed as to the cause of the ordinary variability, and supports the same, that this latter derives from the reproductive system, which is so sensitive to every change in the living conditions that it is often quite incapable, or at least for its proper function, of producing offspring conforming to the elder form, is rendered incapable. Now the hybrids formed in the first generation all descend from species whose reproductive systems have in no way been impaired, except in species long cultivated, and are not variable; but hybrids themselves have a seriously impaired reproductive system, and their offspring are very variable. that it is thereby rendered quite incapable, or at least rendered incapable of its proper function of producing offspring conforming to the parental form. Now the hybrids formed in the first generation all descend from species whose reproductive systems have in no way been impaired, except in species long cultivated, and are not variable; but hybrids themselves have a seriously impaired reproductive system, and their offspring are very variable. that it is thereby rendered quite incapable, or at least rendered incapable of its proper function of producing offspring conforming to the parental form. Now the hybrids formed in the first generation all descend from species whose reproductive systems have in no way been impaired, except in species long cultivated, and are not variable; but hybrids themselves have a seriously impaired reproductive system, and their offspring are very variable.
But let us return to the comparison between mongrels and hybrids. GARTNER maintains that mongrels are more inclined than hybrids to revert to one of the parent forms ; but this difference, if correct, is certainly only a gradual one. GARTNER further emphasizes that when two species, although closely related, are crossed with a third whose hybrids differ widely, while when two very different varieties of one species are crossed with another species,[p. 303]whose mongrels are not very different from each other. However, as far as I am able to see, this result is based on only one test and seems to be the opposite of the experiences that KÖLREUTER made in several tests.
These are only the essentially insignificant differences which GÄRTNER was able to mediate between hybrids and hybrids of the plants. But also the resemblance of the hybrids and mongrels, and in particular the hybrids that arose from closely related species with their elders follows after GÄRTNER the same laws. When two species are crossed, one of them sometimes shows a preponderant power of impressing a resemblance on the hybrid, and so, I believe, with plant varieties. In animals, it is true, often one variety possesses this predominant power over another. The two kinds of hybrid plants from an alternating cross are usually very alike, and so it is with the two kinds of hybrids from an alternating cross. Hybrids, as well as mongrels, can be reverted to either of the two parent forms by repeatedly crossing them in successive generations with one of their parent forms.
These various remarks are evidently applicable to animals; but here the subject becomes exceedingly complicated, partly owing to the existence of secondary sexual characters, and partly more particularly owing to the power usually predominant in either sex of impressing its image on the offspring, just as much where the question is of crossing of species as where where it is a matter of varieties among one another. I think e.g. B., that those writers are right who maintain that the donkey is so preponderant over the horse that as a result both the hinny and the mule resemble the donkey more than the horse; but that this preponderance is more pronounced in the male than in the female donkey, hence the mule as the hybrid of the donkey-stallion and horse-mare[p. 304]Donkey more like than the mule, which has the horse for father and a donkey for mother.
Some writers have laid much weight on the fact that among animals it is only in mongrels that such closely resemble one of their elders; but it can be shown that this is also the case with hybrids, though less frequently than with mongrels. As to the cases I have collected of crossed animals closely resembling one of the two elders, this resemblance seems to be chiefly confined to characters monstrous and sudden in their way, such as albinism, melanism, lack of horns, and absence of the tail and preponderance of the fingers and toes, therefore they have no connection with the traits slowly developed by breeding. Accordingly, there will also be cases of abrupt reversion to one of the two older types in mongrels, which are descended from varieties which are often sudden and semi-monstrous in character, than hybrids which are descended from slowly and naturally formed species. Overall, however, I agree with Dr. PROSPER L UCAS , who, after examining an immense body of facts in animals, concludes that the laws of resemblance between children and parents are the same, whether both parents differ more or less, whether they are one or whether they belong to different varieties or entirely different species.
Apart from the question of fertility and sterility, in all other respects there seems to be a close resemblance of behavior between hybrids and mongrels. On the supposition that species were separately created, and that varieties were first developed by secondary laws, such similar behavior would seem a most strange fact. But if one proceeds from the view that there is no essential difference between species and varieties, then it is perfectly consistent with it.
[p. 305]
Summary of the chapter.) First crosses, both between sufficiently distinct forms to be considered varieties, and between their hybrids, are very often, but not always, sterile. This sterility takes place in all degrees, and is often so insignificant that the two most experienced experimenters who ever lived, when attempting to classify the forms by it, sometimes arrived at diametrically opposed conclusions. Sterility is of innate variability in individuals of the same species, and is exceedingly susceptible to favorable and unfavorable influences. The degree of sterility does not strictly conform to systematic affinity, but depends on some peculiar and intricate laws. It is usually unequal, and often very unequal when the same two species are alternately crossed.
In the same way, as in branching trees the ability of one species or variety to take root in another is connected with mostly unknown differences in their vegetative systems, so in cross-breeding the greater or lesser facility of one species to fertilize itself with another is due to unknown differences caused by their reproductive systems. There is therefore no longer any reason to suppose that by the nature of each species a different degree of sterility has been specially concocted in order to prevent their mutual crossing and mingling--there is reason to suppose that each species of wood is a different and somewhat analogous degree of difficulty in grafting to other ways of hitting has been included to prevent
The sterility of the first crosses between pure species with perfect reproductive systems seems to depend on several causes: in some cases mostly on premature spoilage of the embryo. The sterility of[p. 306]Hybrids with imperfect reproductive systems, and those where that system as well as the whole organization has been disturbed by the fusion of two species into one, seem to correspond closely to that sterility which so often afflicts pure species when their natural conditions of life have been disturbed . This way of looking at things is supported by another kind of parallelism, namely, the crossing of forms which differ only slightly from one another promotes the vigor and fertility of the offspring, just as small changes in the external conditions of life are beneficial for the health and fertility of all organic beings. It's not surprising that the degree of difficulty in fertilizing two species together, and the degree of sterility of their hybrids, generally correspond, though arising from different causes; for both depend on the degree of any difference between the crossed species. Nor is it surprising that the ease of effecting a first cross, the fertility of the resulting hybrids, and the power of mutual grafting, though this latter apparently depends on widely different causes, all parallel to some extent the systematic relationship the forms used in the experiments; for "systematic affinity" is intended to express all sorts of similarities between species. although they arise from different causes; for both depend on the degree of any difference between the crossed species. Nor is it surprising that the ease of effecting a first cross, the fertility of the resulting hybrids, and the power of mutual grafting, though this latter apparently depends on widely different causes, all parallel to some extent the systematic relationship the forms used in the experiments; for "systematic affinity" is intended to express all sorts of similarities between species. although they arise from different causes; for both depend on the degree of any difference between the crossed species. Nor is it surprising that the ease of effecting a first cross, the fertility of the resulting hybrids, and the power of mutual grafting, though this latter apparently depends on widely different causes, all parallel to some extent the systematic relationship the forms used in the experiments; for "systematic affinity" is intended to express all sorts of similarities between species. that the ease of effecting a first cross, the fertility of the resulting hybrids, and the power of mutual grafting, though this latter evidently depends on widely different causes, all parallel to some extent the systematic affinity of the forms observed in the experiments came into use; for "systematic affinity" is intended to express all sorts of similarities between species. that the ease of effecting a first cross, the fertility of the resulting hybrids, and the power of mutual grafting, though this latter evidently depends on widely different causes, all parallel to some extent the systematic affinity of the forms observed in the experiments came into use; for "systematic affinity" is intended to express all sorts of similarities between species.
First crosses between forms which are considered varieties, or at least sufficiently different from each other to be called, and their hybrids, though common, are not (as has so often been mistakenly asserted) invariably fertile. Yet this ordinary and perfect fertility is not strange when we remember how easily we fall into a circular argument concerning varieties in the state of nature, and when we recall that the greater number of varieties are produced by culture by breeding merely by external differences and not according to such[p. 307]produced in the reproductive system. In all other respects, except fertility, there is generally a very close resemblance between hybrids and mongrels. Finally, the facts recently enumerated in this chapter, notwithstanding our utter ignorance of the real causes of sterility, seem to me not to contradict, but in many respects to accord with the view that there is no fundamental difference between species and varieties.
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