Conederation

“ The recessives are pure like those of the last generation, but the dominants can, by further self-fertilization and cultivation of the seeds produced, be again shown to be made up of pure dominant and crossbreds in the same proportion of one dominant to two crossbreds. ”

“ Both extremes seem to be barred in nature—namely, the inter-breeding of stocks so diverse in structure and quality as to be what we call "distinct species," and again the inter-breeding of individuals of the same immediate parentage or near cousinship. What seems to be favoured by the natural structure and qualities of the plant or the animal is that it shall only breed within a certain group—the species—and shall within that group avoid constant self-fertilization or fertilization by near cousins. ”

“ In some cases, I do not doubt that the intercrossing of species, aboriginally distinct, has played an important part in the origin of our domestic productions. When in any country several domestic breeds have once been established, their occasional intercrossing, with the aid of selection, has, no doubt, largely aided in the formation of new sub-breeds; but the importance of the crossing of varieties has, I believe, been greatly exaggerated, both in regard to animals and to those plants which are propagated by seed. ”

“ But do such inter-specific hybrids ever give rise to populations that deserve to be called species?
It must be remembered that transition zone populations grade in every direction into the pure populations between which they are hybrid. Any portion of such a hybrid population is different biometrically from any other portion of that population. The genes available at one point in the transition zone are not equally available at every other point in the zone. There is no common heredity within the population. ”

“ It is also surprising that hardly anything has been done, in a region where everything is so favorable, to improve from themselves the races which have already become adapted to the medium and been molded by it. Cross-breeding, although it is highly esteemed, has been tried only in a limited number of estancias, and the selection of the best native stock has not been seriously attempted on any of the estates that I have observed. ”

“ The general law of the effect of breeding may therefore be graphically represented by the following diagram, in which the absciss A B represents the level of average result, distance on this absciss from the middle point a represents the divergence of crossing varieties, and ordinates positive and negative represent the result of crossing, whether good or bad. Further, the middle point a represents no divergence or identical individuals, the distance b b individual differences, c c divergence constituting slight varieties, d d strong varieties, f f races, and g g species. ”

“ But how can the process of progressive divergence begin, when slight varieties are even more fertile by cross-breeding than by close breeding? Is it not evident that, with every generation,245 the slight varieties would cross-breed with one another and with the parent stock, and thus all varietal differences would be funded into a common stock, and the type would be preserved unchanged? ”

“ The outstanding fact that emerges prominently from the study of the local forms is that when two definite types, nearly allied, and capable of interbreeding with production of fertile offspring, meet together in the region where their distributions overlap, though intergrades are habitually found, there is no normally or uniformly intermediate population occupying the area of intergradation. Such phenomena as these must, I think, be admitted to have great weight in any attempt to construct a theory of evolution. ”

“ Simple cultures are those in which a single species is used. The resulting group is a family, and the competition is between like individuals. In such cultures, the problem of the factors in competition is reduced to its simplest terms. Mixed cultures are based upon two or more species, and the problem is correspondingly complicated. As a rule, all the seeds have been sown at the same time in both simple and mixed cultures, but it has been found desirable to make some heterochronous cultures, in which seeds are also sown after the plants have appeared. ”

“ In the case of varieties of the same species, the struggle will generally be almost equally severe, and we sometimes see the contest soon decided: for instance, if several varieties of wheat be sown together, and the mixed seed be resown, some of the varieties which best suit the soil or climate, or are naturally the most fertile, will beat the others and so yield more seed, and will consequently in a few years quite supplant the other varieties. ”

“ Hybrids
The naming of cultivars derived from crosses between two or more different species, belonging to the same or different genera, involves rather special problems. By "derived from" I mean not only the first generation from a species cross, but all subsequent generations and back-crosses with the original parents or with members of the first or later generations. Any cultivar which is the progeny of a species cross, however remote, comes, for the purpose of the Code, under the heading of a hybrid and its naming is subject to definite rules. ”

“ 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. ”

“ The general limitation of species to certain areas, suggests the idea that each of them, with all their varieties, have sprung from a common parent, and have spread in various directions from a common centre. The frequency also of the grouping of genera within certain geographical limits, is in favour of the same law, although the migration of species may sometimes cause apparent exceptions to the rule, and make the same types appear to have originated independently at different spots. ”

“ If it can be shown to be almost invariably the case, that a region, of which most of its inhabitants are closely related to, or belong to the same genera with the species of a second region, has probably received at some former period immigrants from this other region, my theory will be strengthened; for we can clearly understand, on the principle of modification, why the inhabitants of a region should be related to those of another region, whence it has been stocked. ”

“ Some few families, many sub-families, very many genera, and a still greater number of sections of genera, are confined to a single region: and it has been observed by several naturalists that the most natural genera, or those genera in which the species are most closely related to each other, are generally confined to the same country, or, if they have a wide range, that their range is continuous. ”

“ Before leaving the question of vicinism and its bearing on the general belief of the instability of varieties, which when tested with due care, prove to be quite stable, it may be as well to consider the phenomena from another point of view. Our present knowledge of the effects of crosses between varieties enables us to formulate some general rules, which may be used to calculate, and in some way to predict, the nature of the impurities which necessarily attend the cultivation of allied species in close vicinity. ”

“ We will come to that some day in the future, maybe—not in our lifetime, but we will have hybrid varieties, because, after all, our great improvements that have come in most of our plants, in corn and in wheat, and in other plants, have come through the mixing of the genes, or the characters that we have differing between species. ”

“ Long experience has shown that very close breeding of the same variety for a long time fixes the kind but weakens the stock, especially in fertility, while judicious crossing of varieties strengthens the stock, increasing its fertility, and especially producing plasticity or variability. Therefore breeders, if they wish to preserve a valuable variety, breed close; but, if they wish to make new varieties, cross-breed. But we have already seen that species are usually cross-sterile. ”

“ The degree of sterility of first crosses and of hybrids runs, to a certain extent, parallel with the systematic affinity of the forms which are united. “For species belonging to distinct genera can rarely, and those belonging to distinct families can never, be crossed. The parallelism, however, is far from complete; for a multitude of closely allied species will not unite, or unite with extreme difficulty, whilst other species, widely different from each other, can be crossed with perfect facility. ”

“ The central principle is that when the seed produced from a cross between two different species is planted, the progeny breaks up into well-defined groups. A certain percentage of the plants resemble one of the parents, a smaller percentage are like the other parent, and the rest seem to be a blend of both parents. These intermediates will not breed true to themselves, however; if seed from them is planted the progeny will split up into groups, showing the same percentages as the first generation to which they belonged. ”

“ Local forms may be sharply differentiated from the corresponding populations of other localities or they may be connected with them by numbers of intermediates. Not rarely also we find a fact which has always seemed to me of special significance, that the peculiarity of the local population or colony may show itself in a special liability to variation, and this variability may show itself in one of many degrees, either in the constant possession of a definite aberration, in a dimorphism, or in an extreme polymorphism. ”

“ As respects identical species, interchange is the only thing that, on our views of what a species is, will explain the occurrence of the same species here and there. But as to genera, I do not yet feel free to assume an interchange, or a former continuity of land, between two widely separated regions on account of their having identical genera or closely related species. ”

“ All development, as all growth, is by accretion or assimilation. The assimilating force is, if you will, in the germ, but the matter assimilated comes and must come from abroad. Every herdsman knows it, and knows that to rear his stock he must supply them with appropriate food; every husbandman knows it, and knows that to raise a crop of corn, he must plant the seed in a soil duly prepared, and which will supply the gases needed for its germination, growth, flowering, boiling, and ripening. ”

“ It is on this conception that the almost universal belief rests, that varieties are distinguished, as such, from species by their inconstancy. Now I do not deny the phenomenon itself. The impurity of seeds and cultures is so general and so manifest, and may so easily be tested by every one [199] that it cannot reasonably be subjected to any doubt. It must be conceded to be a fact, that varieties as a rule revert to their species under the ordinary circumstances of commercial culture. ”

“ Hence the fact that the most distinct domestic varieties are, with rare exceptions, perfectly fertile when crossed, and produce fertile offspring, whilst closely allied species are, with rare exceptions, more or less sterile, is not nearly so formidable an objection as it appears at first to the theory of the common descent of allied species. ”

“ Exporting of material of the variety, which enables the propagation of the variety, into a country which does not protect varieties of the plant genus or species to which the variety belongs, except where the exported material is for final consumption. ”

“ The book, like the papers of Allen, Ridgway, Gulick, and others, shows the value of isolation or segregation in special areas as a factor in the origination of varieties and species, the result being the prevention of interbreeding, which would otherwise swamp the incipient varieties.
Here might be cited Delbœuf’s law: [238]
“When a modification is produced in a very small number of individuals, this modification, even were it advantageous, would be destroyed by heredity, as the favored individuals would be obliged to unite with the unmodified individuals. ”

“ I hope that I have succeeded in showing that the difference between elementary species, or, as they are often called, smaller or subspecies, on the one hand and varieties on the other, is quite a marked one. However, in order to recognize this principle it is necessary to limit the term variety, to those propagating themselves by seed and are of pure and not of hybrid origin. ”

“ I will show you this method practically at some fixture time, for by these means we can procure all sorts of fruit; only we must remember, that we can only graft a tree with one of the same natural family; thus, we could not graft an apple on a cherry-tree, for one belongs to the apple tribe, and the other to the plum tribe."
"Do we know the origin of all these European fruits?" asked the inquiring Ernest. ”

“ The variety is sexually isolated from the parent stock by cross-sterility, and therefore all its peculiarities, however trivial, are preserved by true breeding. Cross-breeding is necessary to make species, but true breeding preserves them. Cross-breeding tends ever to make varieties, but immediately destroys them again. This constant forming and swamping, separating and again merging of varieties, like mixing of dough, makes the whole mass (stock) more and more plastic and subject to variety. ”