Half-breeding

“ If is well known to stock breeders that the closer animals are bred in-and-in, that is, the nearer the relationship of father and mother, the more prominently the traits of the parents appear in their children and become fixed in the breed. It is evident that in the earliest epoch of the human family, the closest inter-breeding must have prevailed without restriction, as it does in every species of the lower animals. ”

“ It is well known that too close breeding, or consanguineous breeding, or breeding in and in, as it is variously called, if continued long, has a bad effect on the offspring, weakening the stock, while judicious crossing of varieties within certain limits of difference has a good effect, strengthening the stock and increasing its fertility. ”

“ Hence this difference in habituation may well account for the different action of the reproductive organs when domestic races and when species are crossed. It is a nearly analogous fact, that most domestic races may be suddenly transported from one climate to another, or be placed under widely different conditions, and yet retain their fertility unimpaired; whilst a multitude of species subjected to lesser changes are rendered incapable of breeding. ”

“ Breeders have carried this principle so far as frequently to advocate the growing of plants in the environment most likely to produce the change desired, as, for instance, cultivating tall plants like twining beans in the north or at high altitudes if it is desired to produce a dwarf type or, vice versa, breeding the plants in the south and at a low altitude if a giant or tall type is desired. ”

“ Beardless plants, however, produced in some cases only beardless offspring, in other cases both beardless and bearded offspring in the proportion of three of the former to one of the latter. Out of every three beardless plants only one was found to breed true, whilst two gave a mixed progeny. It appears therefore that in the second generation some of the types which occur breed true, whilst others do not. Some of the true breeding 46 individuals can be picked out at sight, for instance, those with long or short bearded ears. ”

“ The most perfect degree of fertility is obtained when the stigma of one form is fertilised by the pollen taken from stamens of a corresponding height. On the other hand, when the union is, as Mr. Darwin states, illegitimate, that is, when the pollen is taken from stamens not corresponding in length to the style, more or less complete sterility ensues in the progeny, sometimes even utter infertility, such as happens when two distinct species are crossed, so that, in point of fact, the offspring of these illegitimate unions correspond almost precisely to hybrids. ”

“ He also informs me that pure white beasts of either sex are so well known by experienced breeders to be comparatively infertile together, that they are never used for breeding purposes, so that "in some parts of the country, where a tendency to sterility had become so confirmed in the white race that they utterly died out," only the coloured breeds are now to be found. ”

“ This now familiar term, proposed by Darwin, was suggested by the operations of breeders in the development and fixation of races for man's use or fancy—breeding in each generation those individuals only in which the desired points are apparent and predominant: in the seed-bed, by rigidly destroying all plants which do not show some desirable variation, breeding in and in from these with strict selection of the most variant form in the particular line or lines, until it becomes fixed by heredity, and as different from the primal stock as the conditions of the case allow. ”

“ The task of the practical breeder who seeks to establish or fix a new variety produced by cross-breeding in a case involving two variable characters is simply the isolation and propagation of that one in each sixteen of the second generation offspring which will be pure as regards the desired combination of characters. ”

“ But for breeding they are found utterly useless; for though they may themselves be uniform in character, they yield during many generations astonishingly diversified offspring. The breeder is driven to despair, and concludes that he will never form an intermediate race. But from the cases already given, and from others which have been recorded, it appears that patience alone is necessary; as Mr. Spooner remarks, “nature opposes no barrier to successful admixture; in the course of time, by the aid of selection and careful weeding, it is practicable to establish a new breed.” ”

“ If even in such minute particulars as these, hereditary transmission may be distinctly seen, it becomes the breeder to look closely to the "like" which he wishes to see reproduced. Judicious selection is indispensable to success in breeding, and this should have regard to every particular—general appearance, length of limb, shape of carcass, development of chest; if in cattle, the size, shape and position of udder, thickness of skin, "touch," length and texture of hair, docility, &c., &c. ”

“ That normal development or growth by cell division is advantaged by cross-fertilization may mean that the cells divide more readily and normally when they contain protoplasmic 'elements' of a proper degree of diversity than when they have only one kind of protoplasm, as would happen in narrow inbreeding, and also when cross-breeding is too wide for the intimate cooperation required for true fertilization. ”

“ In the breeding of animals the closest inbreeding is frequently resorted to in order to improve the stock, and many examples can be given of the closest possible inbreeding for generations without apparent detriment, but it is universally admitted that the animals selected for such inbreeding must be sound constitutionally, and free from disease. After a certain number of generations however, degeneration apparently sets in. The number of generations through which inbreeding may be carried varies with the species, and the purpose for which the animals are bred. ”

“ Any migrant tends to arrive on breeding grounds relatively late, hence migrants ordinarily have higher index values than do residents.
2. Colonial breeding. The strong synchrony of colonially-breeding species tends to move the modal egg-date toward the time of inception of breeding; as a result colonially-breeding species probably have lower index values than they would have if not colonial.
3. Single-broodedness. Species having only one brood per season tend to have shorter seasons than double-brooded species, and their index values tend to be lower than those of double-brooded species. ”

“ Sexual reproduction can also increase the differences between individuals, because constant cross-breeding must necessarily and repeatedly lead to a combination of forces which tend in the same direction, and which may determine the constitution of any part of the body. If, for instance, the same part of the body is strongly developed in both parents, the experience of breeders tells us that the part in question is likely to be even more strongly developed in the offspring ”

“ THE BEARING ON BREEDING Since the endocrine glands are so subtly sensitive and responsive to environment, and are at the same time so intimately concerned in the process of inheritance—a law which sums up their influence upon resemblance and variation in animals—there is no need to stress their importance for the practical science and art of good breeding, eugenics. ”

“ From a single mother he raised a great diversity of forms, some which fed up rapidly and passed through their development without assuming certain stages, and others which were, as he called them, "laggards," moulting more times than their brethren and developing at a much slower rate. It is greatly to be hoped that such a case may be critically investigated by analytical breeding. ”

“ Heterogone Trimorphism, in which there is a mid-length as well as a long and a short set of stamens and style; occurs in Lythrum Salicaria and some species of Oxalis.
342. There must be some essential advantage in cross fertilization or cross breeding. Otherwise all these various, elaborate, and exquisitely adjusted adaptations would be aimless. Doubtless the advantage is the same as that which is realized in all the higher animals by the distinction of sexes.
§ 2. ACTION OF POLLEN, AND FORMATION OF THE EMBRYO.
343. Pollen-growth. ”

“ But neither in the case of a breed which has been contaminated by a single cross, nor when, in the attempt to form an intermediate breed, half-bred animals have been matched together during many generations, can any rule be laid down how soon the tendency to reversion will be obliterated. It depends on the difference in the strength or prepotency of transmission in the two parent-forms, on their actual amount of difference, and on the nature of the conditions of life to which the crossed offspring are exposed. ”

“ When we remember the large number of distinct characters which are presented by the very numerous varieties of any of our cultivated plants, we arrive at an understanding of the possibilities of improvement which the field of hybridization affords, yet I doubt if many of us have even then an adequate conception of the possibilities. Possibly I may make this more clear by an illustration from my timothy breeding experiments. ”

“ These great difficulties in the way of selecting extremes for vegetative propagation are of course met with everywhere. They impede the work of the breeder to such a degree, that but few men are able to surmount them. Breeding new varieties necessitates the bending of every effort to this purpose, and a clear conception of [757] the manifold aspects of this intricate problem. These fall under two heads, the exigencies of practice, and the physiologic laws of variability. ”

“ In other cases it is only possible to pick out the individual plants which breed true by growing their seed and observing how it behaves. If it produces progeny all of which are like the plant from which the seed was obtained, that plant is a fixed type and will breed true continuously in the future. The final result of the experiment was to obtain in three years from the time the cross was made, four fixed types which subsequent experience has shown breed true continuously, a long eared bearded type, a short eared beardless type, a long eared beardless type and a short eared bearded type. ”

“ Yet with all this advance in our understanding, the methods of breeding that can be recommended for the use of practical breeders have changed but little in the last twenty years, the greatest change being primarily in the greater surety with which we now make recommendations. ”

“ It is calculated that if all survived, after ten breeding seasons, the progeny of an original female might have increased to a population of more than 97,000, under the climatic conditions of eastern Kansas, permitting attainment of breeding maturity late in the second year of life. ”

“ Again, with respect to the fertility in successive generations of the more fertile hybrid animals, I hardly know of an instance in which two families of the same hybrid have been raised at the same time from different parents, so as to avoid the ill effects of close interbreeding. On the contrary, brothers and sisters have usually been crossed in each successive generation, in opposition to the constantly repeated admonition of every breeder. And in this case, it is not at all surprising that the inherent sterility in the hybrids should have gone on increasing. ”

“ These are the two fundamental qualities of animals and plants of which the breeder must avail himself in order to produce new forms. The theoretical principle of breeding is, indeed, extremely simple, but in detail the practical application of this simple principle is difficult and immensely complicated. A thoughtful breeder, acting according to a definite plan, must understand the art of correctly estimating, in every case, the general interaction between the two fundamental qualities of heirship and mutability. ”

“ In the latter case the hybrids, although possibly produced without any appearance of infertility on the part of their parent species, nevertheless prove more or less infertile among themselves, and also with members of either parent species.
3. The degree of both kinds of infertility varies in the case of different species, and in that of their hybrid progeny, from absolute sterility up to complete fertility. Thus, to take the case of plants, “when pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exerts no more influence than so much inorganic dust. ”

“ Several considerations, however, render the fertility of domestic varieties less remarkable than at first appears. It can, in the first place, be clearly shown that mere external dissimilarity between two species does not determine their greater or lesser degree of sterility when crossed; and we may apply the same rule to domestic varieties. In the second place, some eminent naturalists believe that a long course of domestication tends to eliminate sterility in the successive generations of hybrids, which were at first only slightly sterile ”

“ The fertility once re-established, the sterility does not return in the later progeny, a fact strongly suggestive of segregation. Now if the sterility of the cross-bred be really the consequence of the meeting of two complementary factors, we see that the phenomenon could only be produced among the divergent offspring of one species by the acquisition of at least TWO new factors ”

“ It is shown that while the continued selection, over a period of years, of highly fecund females failed to bring about any change in average fecundity of the strain used, this character must nevertheless be inherited since pedigree lines have been isolated which uniformly breed true to definite degrees of fecundity. ”