Darwin's On the Origin of Species. Daniel Duzdevich
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strong propensity for inheritance is known by breeders, whose fundamental belief is that “like produces like.” (Only theoretical writers have thrown doubt on this principle.) When a commonly occurring deviation is observed in both parent and offspring, it may result from the same cause acting on both. But when a very rare deviation due to some extraordinary combination of circumstances appears, say, once in several million individuals all apparently exposed to the same conditions, and it reappears in an offspring, the mere doctrine of chance almost compels us to attribute its reappearance to inheritance. Everyone has heard of albinism, prickly skin, hairy bodies, and other such peculiar characteristics reappearing in several members of the same family. If strange and rare deviations really are heritable, then surely commonplace deviations are also heritable. Perhaps the correct view is to take inheritance of every characteristic as the rule and non-inheritance as the anomaly.
The laws of inheritance are unknown. It is unknown why some given peculiarity of individuals within the same species, or of individuals among different species, is sometimes inherited and sometimes not, why a child reverts to characteristics found in a grandparent or more remote ancestor, or why some peculiarities are inherited in a gender-dependent manner.2 Peculiarities appearing in the males of domestic breeds are often transmitted exclusively, or more strongly, to male progeny. A more important rule is that the age at which a peculiarity first appears tends to be the same in the parent and in its offspring (although sometimes earlier in the offspring). In many cases this cannot be otherwise. For example, the inherited peculiarities of cattle horns can appear only as the offspring mature, and peculiarities in the silkworm are known to appear at the corresponding caterpillar or cocoon stage. Hereditary diseases and some other examples suggest that this rule is generally applicable: even when there is no apparent reason for a peculiarity to appear at a particular stage, it tends to appear in the offspring at the same period of development as in the parent. This is very important to illuminating the rules of embryology. These remarks are, of course, confined to the first appearance of a peculiarity and not to its primary cause, which may have acted on the egg or sperm. If the offspring of a short-horned cow and a long-horned bull develops long horns, then it’s clearly due to the sperm.
Naturalists often argue that when domestic varieties run wild, their characteristics gradually but surely revert to those found in the original stocks, and that, consequently, deductions drawn from domestic varieties cannot be applied to species in nature. I have tried without success to find the decisive facts on which this statement is so often and so boldly made; it would be very difficult to prove, because many established domestic varieties could not possibly survive in the wild. In many cases we do not know what the original stock was and could not tell whether or not reversion had ensued. It would also be necessary to turn loose only one variety to avoid the effects of intercrossing. Nevertheless, varieties sometimes do partially revert to the parental form. For example, if various strains of cabbage were cultivated in very poor soil for many generations, they would probably revert wholly or largely to the wild stock. (However, some effect would have to be attributed to the direct action of the poor soil.) Whether or not the experiment would succeed is not particularly important to the argument, because the experiment necessarily alters the environment. If a strong tendency for reversion – that is, a loss of acquired characteristics under constant conditions in a large population so that free crossing, by blending, checks slight deviations of structure – were demonstrated in domesticated varieties, I would grant that nothing deduced from domestic varieties would apply to species. But there is not a shadow of evidence in favor of this view. To assert that we could not breed cart and racehorses, long- and short-haired cattle, and poultry of various breeds, and cultivate edible vegetables for an almost infinite number of generations is contrary to all experience. When the environment changes in nature, variations and reversions probably do occur, but natural selection, as will be explained, determines how far such new characteristics are preserved.
As already mentioned, there is less uniformity of character among individuals of a domestic variety than among individuals of a true species. Also, domestic varieties of the same species often have a monstrous character, by which I mean that although they differ in some minor respects from one another and members of the same genus, they often differ extremely in some one part. With these exceptions and that of the perfect fertility of crossed varieties (discussed later), domestic varieties of the same species differ from one another in a manner similar to the way closely related species of the same genus differ in the wild. There are very few domestic varieties of plant or animal that have not been classified by some competent judges as just varieties and by others as descendants of distinct parent species; if there were any significant distinction between domestic varieties and species, this source of doubt would be less common. Contrary to frequently made assertions, I think domestic varieties differ from one another in generic characteristics,3 which naturalists disagree in defining because all such valuations are currently empirical. Given the following examination of the origin of genera, there is no reason to often expect generic differences in domesticated organisms.
Attempts to estimate the amount of structural difference between domestic varieties of the same species are hampered by our ignorance of whether they have descended from one parent species or several; it would be interesting to clear up this problem. For example, if it were shown that the greyhound, bloodhound, terrier, spaniel, and bulldog, which propagate their kind truly, are derived from a single species, the supposed immutability of the many closely related natural species (such as the foxes) would be brought under considerable doubt. I do not believe that all dog breeds have descended from one wild species (see below),4 but there is tentative or even strong evidence that some other domestic varieties have.
Humans are often assumed to have chosen for domestication those plants and animals that possess an extraordinary inherent tendency to vary and to withstand diverse climates. Although such capacities have added significantly to the value of many domesticated productions, how could primitive humans have possibly known when first taming an animal that it would vary in succeeding generations and endure other climates? The limited variability of the ass and the guinea fowl, and the low tolerance for warmth by the reindeer and for cold by the common camel did not prevent their domestication. If plants and animals equal in number and belonging to equally diverse classes and regions to existing domesticated organisms were taken from the wild and bred for an equal number of generations under domestication, they would vary on average as much as the parent species of already domesticated organisms have varied.
I think it is impossible to ascertain with complete certainty whether established domesticated plants and animals have descended from one or multiple species. Those who believe in the multiple origin of domestic animals argue mainly that ancient records, especially on the monuments of Egypt, reveal a great diversity of breeds, some of which resemble or are identical to existing ones. Even if this were found to be more strictly and generally true than I believe is the case, it suggests only that some of our breeds originated there four or five thousand years ago. Based on Mr. Horner’s research, civilization advanced enough to manufacture pottery probably existed in the Nile valley thirteen or fourteen thousand years ago; it is not known how long before these ancient periods peoples like those of Tierra del Fuego or Australia, who possess a semidomesticated dog, may have existed in Egypt.
I think the whole subject must remain vague. Nevertheless, without going into details – but based on geographic and other considerations – I think it is likely that domestic dogs have descended from several wild species. I cannot form an opinion with respect to goats and sheep. Information about the habits, voice, constitution, and other features of humped Indian cattle, communicated to me by Mr. Blyth, indicate that it descended from a different stock than European cattle, which, in turn, have more than one parent, according to several judges. And for reasons I cannot cover here, I am doubtfully inclined to believe, in opposition to several authors, that all the varieties of horse have descended from one wild stock. Mr. Blyth – whose opinion I value highly, drawn as it is from his large and varied stores of knowledge – thinks that all poultry breeds have proceeded from the common wild Indian fowl. Duck and rabbit breeds, which differ considerably from one another in structure, have all descended from the common wild duck and rabbit.
Some authors carry the doctrine of plural descent to an absurd extreme, believing that every variety