Inbreeding

“ Results of inbreeding with maize—crossing between different races or genotypes, if not too distantly related, results in a progeny which excels its parents in vitality, whereas crosses between individuals belonging to the same genotype engender no increase of vitality as compared with the parents. ”

“ Not only must the prohibition of incest be older than all breeding of domestic animals from which men could derive experience of the effect of inbreeding upon the characteristics of the breed, but the harmful consequences of inbreeding are not established beyond all doubt even to-day and in man they can be shown only with difficulty. ”

“ Inbreeding is the reverse of outcrossing. It is the practice of mating animals closely related to each other, and it is, within limits, an entirely justifiable means of preserving and intensifying family characteristics. ”

“ Selective inbreeding for a few generations would first produce uniformity, or 'fix the type,' as the expression is, by inducing the hemilytic or retarded stage of evolution, while a too narrow and persistent selection or the segregation of a single line of descent would hasten the decline and eventual destruction of the very type it might be designed to perpetuate. ”

“ Inbreeding simply intensifies whatever is there. It does not necessarily destroy the vitality, but if close inbreeding is practiced long enough, sooner or later some little existing weakness or peculiarity would become intensified and may prove fatal to the strain. ”

“ Normally a child has two parents, four grandparents, and eight great-grandparents. Now, when cousins marry, the great-grandparents of the offspring are reduced to six. The mating of brother and sister cuts the grandparents to two, and the great-grandparents to four. Mating of parent and offspring makes a parent and grandparent identical and likewise eliminates ancestry. Inbreeding means the reduction of the number of branches in the ancestral tree, and this means the reduction of the number of chances to get variation, be they good or bad. ”

“ Very early the savage observed that race mixture improved the quality of the offspring. It was not that inbreeding was always bad, but that outbreeding was always comparatively better; therefore the mores tended to crystallize in restriction of sex relations among near relatives. It was recognized that outbreeding greatly increased the selective opportunity for evolutionary variation and advancement. The outbred individuals were more versatile and had greater ability to survive in a hostile world; the inbreeders, together with their mores, gradually disappeared. ”

“ If you breed from the male and female of the same race, you of course have offspring of the like kind, and if you make the offspring breed together, you obtain the same result, and if you breed from these again, you will still have the same kind of offspring; there is no check. But if you take members of two distinct species, however similar they may be to each other, and make them breed together, you will find a check, with some modifications and exceptions, however, which I shall speak of presently. ”

“ 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. ”

“ There is a class of cases of inheritance, due to the XY chromosomes, that is called sex linked inheritance. It is shown both by mutant characters and characters of wild species.
For instance, white eye color in Drosophila shows sex linked inheritance. If a white eyed male is mated to a wild red eyed female (fig. 35) all the offspring have red eyes. If these are inbred, there are three red to one white eyed offspring, but white eyes occur only in the males. ”

“ It is effected by their action in crossing, by their skill in bringing the right mates together to produce the progeniture they want. But in natural selection who is to supply the breeder's place? Unless the crossing is properly arranged, the new breed will never come into being. What is to secure that the two individuals of opposite sexes in the primeval forest, who had been both accidentally blessed with the same advantageous variation, shall meet, and transmit by inheritance that variation to their successors? ”

“ Suppose then the offspring of any parent to vary in many directions. By cross-breeding among themselves and with the parent stock, these are usually merged in a common type, their differences pooled, and the species remains fixed or else advances slowly by natural selection, along one line, as physical conditions change in geological time. But from time to time there arises a variation in the reproductive organs of some individuals, of such kind that these individuals are fertile among themselves, but sterile or less fertile with other varieties and with the parent stock. ”

“ As offspring tend to resemble grandparents almost as much as parents, and as a line of close-bred ancestry is generally prepotent, so newly-originated varieties have always a tendency to reversion. This is pretty sure to show itself in some of the progeny of the earlier generations, and the breeder has to guard against it by rigid selection. But the older the variety is—that is, the longer the series of generations in which it has come true from seed—the less the chance of reversion: for now, to be like the immediate parents, is also to be like a long line of ancestry ”

“ On the whole, as far as I can judge, new characters are more apt to appear in the males of our domesticated animals than in the females, and afterwards to be either exclusively or more strongly inherited by the males. Finally, in accordance with the principle of inheritance as limited by sex, the appearance of secondary sexual characters in natural species offers no especial difficulty, and their subsequent increase and modification, if of any service to the species, would follow through that form of selection which in my 'Origin of Species' I have called sexual selection. ”

“ But when a breed has been crossed only once by some other breed, the offspring occasionally show for many generations a tendency to revert in character to the foreign breed—some say, for a dozen or even a score of generations. After twelve generations, the proportion of blood, to use a common expression, from one ancestor, is only 1 in 2048; and yet, as we see, it is generally believed that a tendency to reversion is retained by this remnant of foreign blood. ”

“ If you cross two such species with each other, then,—although you may get offspring in the case of the first cross, yet, if you attempt to breed from the products of that crossing, which are what are called HYBRIDS—that is, if you couple a male and a female hybrid—then the result is that in ninety-nine cases out of a hundred you will get no offspring at all ”

“ To understand breeding to-day we must clearly understand the conception of unit characters. We no longer conceive the species, race or variety, as a fixed ensemble of characters. Following De Vries, we now commonly conceive the species or variety to be made up of a certain number of unit characters, that are in large measure associated together by the accident of evolution or breeding and which are separable entities in inheritance. ”

“ The laws governing inheritance are quite unknown; no one can say why the same peculiarity in different individuals of the same species, and in individuals of different species, is sometimes inherited and sometimes not so; why the child often reverts in certain characters to its grandfather or grandmother or other much more remote ancestor; why a peculiarity is often transmitted from one sex to both sexes, or to one sex alone, more commonly but not exclusively to the like sex. ”

“ Distinction between the sterility of first crosses and of hybrids—Sterility various in degree, not universal, affected by close interbreeding, removed by domestication—Laws governing the sterility of hybrids — Sterility not a special endowment, but incidental on other differences—Causes of the sterility of first crosses and of hybrids—Parallelism between the effects of changed conditions of life and crossing—Fertility of varieties when crossed and of their mongrel offspring not universal—Hybrids and mongrels compared independently of their fertility—Summary. ”

“ Now it has always been known that breeders could produce a race of markedly peculiar form or characteristics by selecting the individuals possessing this quality in the highest degree and breeding only from these. The breeder depends upon heredity, variation, and his selection of the individuals from which to breed. Similarly in nature new species have arisen through heredity, variation, and a selection according to the laws of nature of those varying in conformity with their environment. ”

“ 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 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. ”

“ Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause, namely, from close interbreeding. I have collected so large a body of facts, showing that close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual or variety increases fertility, that I cannot doubt the correctness of this almost universal belief amongst breeders. Hybrids are seldom raised by experimentalists in great numbers ”

“ We need not feel surprise at sterility incidentally resulting from crosses between distinct species,—the modified descendants of a common progenitor,—when we bear in mind how easily the reproductive system is affected by various causes—often by extremely slight changes in the conditions of life, by too close interbreeding, and by other agencies. ”

“ In pre-Mendelian times all that was known was that some forms could freely interbreed without diminution of fertility in the product, while others could not. But now we find that, by virtue of segregation, from one and the same pair of parents, or even, in the case of hermaphrodites, from one and the same individual, offspring commonly arises showing among themselves exactly such differences as distinguish species—and very good species too. ”

“ But the sterility which must be obtained by the selective breeder in order to convert his morphological species into physiological species—such as we have in nature—must be quite irrespective of breeding in and in. ”

“ Hence the successive steps in the modification of a species might readily be transmitted in different ways; some of the steps being transmitted to one sex, and some to both; some to the offspring at one age, and some at all ages. Not only are the laws of inheritance extremely complex, but so are the causes which induce and govern variability. ”

“ But it is not as yet proved that a race ever exhibits, when crossed with another race of the same species, those phænomena 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. ”

“ Practically all our views on the subject of "races" and their inter-breeding are derived from our observation of the immense number and range of "races" and "breeds" produced by man—as farmer, fancier, and horticulturist. It has been generally received as a rule, that the various races produced in the farm or garden by breeding from a species, will inter-breed freely, and produce offspring which are fertile. ”

“ For in all higher animals, at least, offspring may possess some of the characteristics originally present in either of two lines of ancestry, and this commingling of such possessions is possible only through sexual reproduction. As has already been seen, in the pairing of chromosomes previous to reduction, the corresponding members of a pair always come together so that in the final segregation each gamete is sure to have one of each kind although whether a given chromosome of the haploid set is of maternal or paternal origin seems to be merely a matter of chance. ”

“ In other cases, in which the breed has not been crossed, but some ancient character has been lost through variation, it occasionally reappears through reversion, so that the parents apparently fail to transmit their own likeness. In all cases, however, we may safely conclude that the child inherits all its characters from its parents, in whom certain characters are latent, like the secondary sexual characters of one sex in the other. ”