Hybridization

“ In general, it is assumed that in hybridization we are dealing merely with characters already present and that new characters which appear are due to the different reactions caused by new associations of unit characters in their mutual effect on one another. ”

“ Among domesticated plants hybridization signifies the reverse of selection, the crossing of varieties which the breeder commonly strives to keep separate. Generalizations to the effect that hybrids as a whole are sterile, variable, weak or vigorous are fallacious, since the results of the crossing depend upon the evolutionary status of the parents. By segregation or inbreeding normal or progressive variation gradually gives place to uniformity and then to mutation, but hybrids between distant types pass at once from the progressive stage to the catalytic. ”

“ Hybridization differs from ordinary crossing only in degree. A species is only a race which has assumed greater fixity. The purposes and results of crossing within the species and of hybridization of different species are essentially alike. The formation of the new individual by the sexual relation of two parents is in itself a species of crossing, giving each new individual in its degree new traits or new combinations. ”

“ Professor Hubrecht also states that 'now for the first time—forty years after the appearance of the 'Origin of Species'—the actual birth of a species has been observed by him.' As I have produced several good species by hybridization, as good as nature herself has produced, and as others have done the same by selection alone, the above sentence is hardly true. But as before stated, hybridization followed by selection is the shortest plan by which valid new species can be produced. ”

“ First comes the choice of the material from the assemblage of species, elementary species and varieties. Hybridization comes next in importance. But even the hybrids of the best parents may be improved, because they are no less subject to Quetelet's law than any other strain. Any large number of hybrids of the same ancestry will prove this, and often the excellency of a hybrid variety depends chiefly, or at least definitely, on the selection of the best individuals. ”

“ He finds that in the process of hybridization there are certain characteristics which are transmitted entire to the offspring, and are termed dominant, others which seem to disappear or become latent in the process, which he terms recessive. ”

“ The occurrence of hybrids in nature explains the difficulty experienced by botanists in deciding on what is a species, and the widely different limitations of the term adopted by different observers in the case of willows, roses, brambles, &c. The artificial process is practically the same in hybridization as in cross-fertilization, but usually requires more care. ”

“ In heterogeneous hybridization or the crossing of distantly related forms, all sorts of results are reached, according to the nature of the forms in question. In few cases does the progeny become mature, and still more rarely does it prove fertile. ”

“ But it is not as yet proved that a race ever exhibits, when crossed with another race of the same species, those phenomena of hybridization which are exhibited by many species when crossed with other species. On the other hand, not only is it not proved that all species give rise to hybrids infertile 'inter se', but there is much reason to believe that, in crossing, species exhibit every gradation from perfect sterility to perfect fertility. ”

“ It would be a matter of vast interest to determine what is the difference in the structure of the sperm-cells of a fertile and of an infertile male hybrid. At present, so far as I know, this has not been done.
So far what I have written applies to hybridization—the inter-breeding of distinct species. A similar but by no means identical subject is that of the inter-breeding of distinct races or varieties of one species, and the production of "mongrels." "Mongrels" are to races what "hybrids" are to species. ”

“ It is said that such a test is to be found in the phenomena of hybridization—in the results of crossing races as compared with the results of crossing species.
So far as the evidence goes at present, individuals, of what are certainly known to be mere races produced by selection, however distinct they may appear to be, not only breed freely together, but the offspring of such crossed races are also perfectly fertile with one another. ”

“ Hybridization, moreover, is successful between varieties of the same species rather than between distinct species. Interspecific crosses are in some cases entirely unproductive, in other cases productive of wholly-sterile, hybrids, and in still other cases productive of semisterile hybrids. When semisterile hybrids are obtainable from an interspecific cross, the phenotype can be kept constant by somatogenic reproduction, but, as we shall see in a later chapter, this kind of reproduction does not counteract senescence, and stock thus propagated usually plays out within a determinate period. ”

“ The process of artificial selection is used in all cases, those varying strains likely to prove useful being preserved, the others destroyed. The word 'crossing' may be advantageously used for the mingling of strains within a species, and 'hybridization' for the breeding together of members of different species. ”

“ The conclusions which others have reached, and upon which de Vries has laid the greatest stress, that quantitative and qualitative characters differ fundamentally in their heritability is supported by our experiments.
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G. THE RÔLE OF HYBRIDIZATION IN EVOLUTION.
The criticism has often been made of modern studies in hybridization that they are really unimportant for evolution because hybridization is uncommon in nature. ”

“ Morris: Mr. Chairman, and Members of the Association: My subject relates to personal experiences with hybridization work. This is work which is to be done more and more by various members of our association, and we are thus to create new species of trees. Nature's whole endeavor is to preserve the mean type among races of organisms. There are mutants among all trees, among the hickories and walnuts, as well as among the peaches and pears. In fact all species undergo mutation. We select the most desirable mutants and we try to fix a given type by grafting and propagating. ”

“ In connection with the last-named feature, the behavior of the individual qualities or unit-characters in hybridizations are being studied with enormous zeal as offering a ready analysis of the action of inheritance. The comparative ease with which hybrid combinations of plants are effected, and the simplicity of the subsequent resolutions in the progeny, render this phase of the subject easy of attack, and results are being obtained, which, if one may judge from recent literature, do not seem available to many writers. ”

“ The farther the study of characters is carried the more we are coming to realize that the appearance of apparently new types following hybridization is due to recombinations of different units which in their reactions give apparently new characters. ”

“ The result of this is that the relative number of dominants and recessives increases in successive generations, whereas the relative number of mixed dominant-recessives decreases, and in a few generations a hybrid race will revert in large part to its parental types if continued hybridization is prevented. ”

“ Perhaps on close inspection you may find in some cases a slight difference, some character being not as fully developed in the hybrid as in the species. But when such is not the case, or where the opportunity for such a closer examination is wanting, a hybrid may easily be taken for a specimen of the pure race. Now take the seeds of your plant and sow them. If you had not supposed it to be hybrid you will be astonished at finding among its progeny some of a wholly different type. You will be led to conclude that you are observing a sudden change in structure such as is usually called a sport. ”

“ The hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced — two classes of facts which are generally confounded together — is by no means strict. There are many cases, in which two pure species, as in the genus Verbascum, can be united with unusual facility, and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. ”

“ And now we see that this same conclusion is independently and strongly fortified by the consideration of the illegitimate unions of trimorphic and dimorphic heterostyled plants. In so complex and obscure a subject as hybridism it is no slight gain to arrive at a definite conclusion, namely, that we must look exclusively to functional differences in the sexual elements, as the cause of the sterility of species when first crossed and of their hybrid offspring. ”

“ The very important and interesting phenomenon of hybridism also belongs to this law of mixed or amphigonous211 transmission. It alone, when rightly estimated, is quite sufficient to refute the prevailing dogma of the constancy of species. Plants, as well as animals, belonging to quite different species, may sexually mingle with one another and produce descendants which in many cases can again propagate themselves, and that indeed either (more frequently) by mingling with one of the two parental species, or (more rarely) by pure in-breeding, hybrid mixing with hybrid. ”

“ A hybrid may be named by placing the names of the parent species or subspecies in alphabetical order, connected by the sign ×; but in hybrids experimentally produced, or in which the sex of the parents is known, the female parent is to be written first, and the sex indicated by the signs ?, ?; Examples. — Carex debilis × virescens ; Digitalis lutea ? × purpurea ?. (b) A hybrid may be named when desirable like a species or subspecies, provided the binomial or trinomial is preceded by the sign ×, designating it as a hybrid. ”

“ As in hybridisation, so with grafting, the capacity is limited by systematic affinity, for no one has been able to graft together trees belonging to quite distinct families; and, on the other hand, closely allied species and varieties of the same species, can usually, but not invariably, be grafted with ease. But this capacity, as in hybridisation, is by no means absolutely governed by systematic affinity. Although many distinct genera within the same family have been grafted together, in other cases species of the same genus will not take on each other. ”

“ I well know the large amount of natural as well as artificial hybridisation that goes on. But, on the other hand, there are so many species which either will not cross at all, or produce sterile hybrids, that, taking a general view of all species together, mutual sterility does become by far the most generally distributed single peculiarity—i.e. is the one peculiarity which, more than any other that can be named, is common to numberless species. ”

“ Secondly, a large number, no doubt the greater number of novelties, are of hybrid origin. Here we may discern two cases. Hybrids may be produced by the crossing of old types, either of two old cultivated forms or newly introduced species, or ordinarily between an old and an introduced variety. Such novelties are excluded from our present discussion. Secondly, hybrids may be produced between a true, new mutation and some of the already existing varieties of the same species. ”

“ Why should there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of hybrids been permitted? to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems to be a strange arrangement. ”

“ Their incompatibility may well be of the same nature as that of the classes in Cardamine pratensis.
Returning now to the problem of inter-specific sterility; we note, as I have said, the absence of contemporary evidence that variation can confer on a variety the power to form a sterile hybrid with the parent species. The considerations based on this want of evidence have for a long while been familiar to all who have discussed evolutionary theories, and it is worth observing the exact reason why the difficulty strikes us now with a new and special force. ”

“ In all the several foregoing points the parallelism is wonderfully close between the effects of illegitimate and hybrid fertilisation. It is hardly an exaggeration to assert that seedlings from an illegitimately fertilised heterostyled plant are hybrids formed within the limits of one and the same species. This conclusion is important, for we thus learn that the difficulty in sexually uniting two organic forms and the sterility of their offspring, afford no sure criterion of so-called specific distinctness. ”

“ Theory of Hybridism.—Charles Darwin was interested in hybridism as an experimental side of biology, but still more from the bearing of the facts on the theory of the origin of species. It is obvious that although hybridism is occasionally possible as an exception to the general infertility of species inter se, the exception is still more minimized when it is remembered that the hybrid progeny usually display some degree of sterility. ”