Given our ignorance of the relationships among organisms, it is not surprising that much remains to be explained about the origin of species. Who can explain why one species is numerous with a wide range while a related species is rare with a narrow range? And yet such questions are important because their answers explain the present state and future modifications and success of all organisms. We know even less about the relationships among the innumerable past inhabitants of the earth. Although much remains obscure, and will long remain obscure, the most deliberate study and dispassionate judgment of which I am capable have dissuaded me from the view commonly held by naturalists, and previously held by me, that each species has been independently created. I am fully convinced that species are mutable and that species within a genus are linearly descended from some usually extinct species, in the same way that a variety of a given species is a descendant of that species. I am also convinced that natural selection has been the main, though not exclusive, means of modification.
1
VARIATION UNDER DOMESTICATION
IN CONSIDERING THE INDIVIDUALS OF A DOMESTICATED plant or animal variety, it is striking that they are generally more diverse than those belonging to varieties or species in the wild. The vast diversity of domesticated organisms, which have varied under many different climates and treatments, suggests that greater variability results from the conditions under which domestication occurs – conditions unlike those encountered by the parent species in the wild. This variability may partly be connected with excess food, as proposed by Andrew Knight. It seems clear that organisms must be exposed to a new environment over several generations for it to cause appreciable variation, and once organization begins to vary, it usually continues to do so for many generations. There is no case of a variable organism ceasing to be variable under domestication. Established domesticated plants such as wheat still often yield new varieties, and animals domesticated long ago are still capable of rapid improvement or modification.
It is disputed whether the causes of variation – whatever they may be – act during the early or late stage of embryonic development or at the instant of conception. Isidore Geoffroy St. Hilaire’s experiments show that unnatural treatment of the embryo causes monstrosities, which cannot be clearly differentiated from mere variations. I strongly suspect that variability is most frequently caused by effects on the egg or sperm before conception, mainly because of the remarkable influence of cultivation or confinement on the functions of the reproductive system, which appear far more susceptible to environmental changes than any other component of organization. Nothing is easier than taming an animal and nothing more difficult than getting it to reproduce in confinement, even when the male and female mate. This is generally attributed to impaired instincts, but many cultivated plants are vigorous yet do not seed. In some cases, minor changes, like a little more or less water at a particular period of growth, determine whether or not a plant will produce seeds. I will not go into the copious details I have collected on this curious subject, but to illustrate the strangeness of the rules that govern the reproduction of captive animals, consider that, with the exception of bears, carnivorous mammals, even from the tropics, breed freely in Britain under confinement, whereas carnivorous birds rarely lay fertile eggs. Many exotic plants have pollen as useless as that of the most sterile hybrids. Some domesticated plants and animals that are otherwise weak and sickly breed freely under confinement; but tame, long-lived, and healthy individuals taken young from the wild may have reproductive systems so seriously affected by unknown causes that they are nonfunctional. Unsurprisingly, then, when the reproductive system actually works under confinement, it does so irregularly, producing offspring that are different from the parents. Finally, some organisms breed under very unnatural conditions – like rabbits and ferrets kept in hutches – demonstrating that their reproductive systems have not been affected. So some organisms withstand domestication and vary only slightly, perhaps hardly more than in the wild.
Sterility is a horticultural nuisance, but variability, the source of all the choicest productions of the garden, shares a cause with sterility. There are many plants (called “sporting” plants by gardeners) that produce single buds or offshoots with novel characteristics, sometimes very different from the rest of the plant. Such buds can be propagated by grafting or other techniques, and sometimes by seed. These “sports” are rare in the wild but common under cultivation. In this case, manipulation of the parent affects a bud or offshoot but not the ovules or pollen. According to most physiologists, however, there is no essential difference between a bud and an ovule in the earliest stages of formation. Therefore, sports show that variability may be largely attributed to the effect on the ovules, pollen, or both by treatment of the parent prior to conception. In any case, these examples demonstrate that variation is not necessarily connected with the act of generation, as some authors have suggested.
Seedlings from the same fruit and young from the same litter sometimes differ considerably from each other even though both parent and offspring have apparently been exposed to the same conditions, as Müller has remarked. This shows how unimportant direct environmental effects are in comparison to the laws governing reproduction, growth, and inheritance. If the influence of environment were direct, then variation would be the same among offspring. Judging the extent to which heat, moisture, light, food, and other factors have an impact on variation is difficult. My impression is that such agents produce very little direct effect on animals, but apparently more on plants. (Mr. Buckman’s recent experiments on plants are valuable here.) When all or nearly all individuals exposed to certain conditions are identically affected, the resultant changes appear to flow directly from the conditions. But in some cases opposite conditions generate similar structural changes. Nevertheless, some slight amount of change may be attributed to direct environmental action, as in certain cases of increased size from greater food intake, altered coloration from particular kinds of food and light, and perhaps the thickness of fur from climate.
Habit also has a deciding influence, as with the flowering period of plants transported from one climate to another. The effect is greater in animals. For example, I find that the wing bones of a domestic duck weigh less and the leg bones weigh more in proportion to the whole skeleton than do those of a wild duck. I presume this results from the domestic duck flying much less and walking more than its wild parent. The large inherited udders of cows and goats in countries where they are habitually milked is another example of the effect of use. There is no domestic animal that in some region does not have drooping ears. As suggested by some authors, this probably results from the disuse of ear muscles, the animals being rarely alarmed.
Many rules regulate variation; some of them can be dimly seen and will be briefly mentioned. Here I will only allude to “correlated growth.” For example, a change in the embryo or larva often entails changes in the mature animal. With monstrosities, correlations between distinct parts are very curious.1 Breeders maintain that long limbs are often accompanied by an elongated head. Some correlations are whimsical; for example, cats with blue eyes are invariably deaf. There are many remarkable cases among plants and animals of coloration and constitutional peculiarities going together. Observations collected by Heusinger suggest that white sheep and pigs are affected differently by poisonous vegetables than individuals with coloration. Hairless dogs have imperfect teeth; long-haired and coarse-haired animals tend to have long or many horns; pigeons with feathered feet have skin between their outer toes; pigeons with short beaks have small feet, and those with long beaks have large feet. If humans select and augment a peculiarity, they will probably unintentionally modify other parts as a consequence of the mysterious rules of correlated growth.
The dimly seen or unknown rules of variation yield infinitely complex and diverse results. Treatises on some established domesticated plants, such as the hyacinth, potato, and even the dahlia, reveal a surprising number of slight structural and constitutional differences between varieties and subvarieties. The whole organization seems to have become plastic, with a tendency to depart somewhat from the parental type.
Variations that cannot be inherited are unimportant to this argument. What nevertheless remains is an endless number of diverse heritable structural deviations of both slight and considerable physiological importance. (Dr. Prosper Lucas’s two-volume work is the best treatment of