5. There is also a physiological distinction between species, namely, that the individuals are sterile with one another, whereas this does not apply to varieties; and though Darwin has labored to break down this distinction by insisting on rare exceptional cases, and suggesting many supposed ways by which varieties of the same species might possibly attain to this kind of distinctness, the difference still remains as a fact in nature; though one not readily available in practically distinguishing species.
These general properties of species will, I think, be admitted by all naturalists as based on nature, and absolutely necessary to the existence of natural history as a science, independently of any hypotheses as to the possible changes of specific forms in the lapse of time. I now proceed to give a similar summary of the laws of the varieties which may exist—always be it observed, within the limits of the species.
1. The limits of variation are very different in different species. There are many in which no well-marked variations have been observed. There are others in which the variations are so marked that they have been divided, even by skilful naturalists, into distinct species or even genera. I do not here refer to differences of age and sex. These in many animals are so great that nothing but actual knowledge of the relation that subsists would prevent the individuals from being entirely separated from one another. I refer merely to the varieties that exist in adults of the same sex, including, however, those that depend on arrest of development, and thus make the adult of one variety resemble in some respects the young of another; as, for instance, in the hornless oxen, and beardless individuals among men. If we inquire as to the causes on which the greater or less disposition to vary depends, we must, in the first place, confess our ignorance, by saying that it appears to be in a great measure constitutional, or dependent on minute and as yet not distinctly appreciable structural, physiological, and psychical characters. Darwin states that Pallas long ago suggested, from the known facts that the seeds of hybrid plants and grafted trees are very variable, the theory that mixture of breeds tends to produce variability; but Darwin does not seem to attach much importance to this, and admits our inability to explain the origin of these differences. [161] We know, however, certain properties of species that are always or usually connected with great liability to variation. The principal of these are the following: 1. The liability to vary is, in many cases, not merely a specific peculiarity; it is often general in the members of a genus or family. Thus the cats, as a family, are little prone to vary; the wolves and foxes very much so. 2. Species that are very widely distributed over the earth's surface are usually very variable. In this case the capacity to vary probably adapts the creature to a great variety of circumstances, and so enables it to be widely distributed. It must be observed here that hardiness and variability of constitution are more important to extensive distribution than mere locomotive powers, for matters have evidently been so arranged in nature that, where the habitat is suitable, colonists will find their way to it, even in the face of difficulties almost insurmountable. 3. Constitutional liability to vary is sometimes connected with or dependent on extreme simplicity of structure, in other cases on a high degree of intelligence and consequent adaptation to various modes of subsistence. Those minute, simply organized, and very variable creatures, the Foraminifera, exemplify the first of these apparent causes; the crafty wolves furnish examples of the second. 4. Susceptibility to variation is farther modified by the greater or less adaptability of the digestive and locomotive organs to varied kinds of food and habitat. The monkeys, intelligent, imitative, and active, are nevertheless very limited in range and variability, because they can comfortably subsist only in forests, and in the warmer regions of the earth. The hog, more sluggish and less intelligent, has an omnivorous appetite, and no very special requirements of habitat, and so can vary greatly and extend over a large portion of the earth. Farther, in connection with this subject it may be observed that the conditions favorable to variation are also in the case of the higher animals favorable to domestication, while it may also be affirmed that, other things being equal, animals in a domesticated state are much more liable to vary than those in a wild state, and this independent of intentional selection. Darwin admits this, and gives many examples of it.
2. Varieties may originate in two different ways. In the case of wild animals it is generally supposed that they are gradually induced by the slow operation of external influences; but it is certain that in domesticated animals they often appear suddenly and unexpectedly, and are not on that account at all less permanent. A large proportion of our breeds of domestic animals appear to originate in this way. A very remarkable instance is that of the "Niata" cattle of the Banda Orientale, described by Darwin in his "Voyage of a Naturalist." These cattle are believed to have originated about a century ago among the Indians to the south of the La Plata, and the breed propagates itself with great constancy. "They appear," says Darwin, "externally to hold nearly the same relation to other cattle which bull-dogs hold to other dogs. Their forehead is very short and broad, with the nasal end turned up, and the upper lip much drawn back; their lower jaws project outward; when walking they carry their heads low on a short neck, and their hinder legs are rather longer compared with the front legs than is usual." It is farther remarkable in respect to this breed that it is, from its conformation of head, less adapted to the severe droughts of those regions than the ordinary cattle, and can not, therefore, be regarded as an adaptation to circumstances. In his later work on animals under domestication, Darwin gives many other instances of the origination of breeds of cattle and other animals in this abrupt and mysterious manner, and without any selection, though he strongly leans to the conclusion that slow and gradual changes are the most frequent causes of variation. It is to be observed, however, that very slow changes are in more danger of being accidentally diverted or obliterated by crossing, and that the first stages of an incipient change may be too unimportant to be permanent.
Many writers on the subject of the Unity of Man assume that any marked variety must require a long time for its production. Our experience in the case of the domestic animals teaches the reverse of this view; a very important point too often overlooked.
3. The duration or permanence of varieties is very different. Some return at once to the normal type when the causes of change are removed. Others perpetuate themselves nearly as invariably as species, and are named races. It is these races only that we are likely to mistake for true species, since here we have that permanent reproduction which is one of the characteristics of the species. The race, however, wants the other characteristics of species as above stated; and it differs essentially in having branched from a primitive species, and in not having an independent origin. It is quite evident that in the absence of historical evidence we must be very likely to err by supposing races to have really originated in distinct "primordial forms." Such error is especially likely to arise if we overlook the fact of the sudden origination of such races, and their great permanency if kept distinct. There are two facts which deserve especial notice, as removing some of the difficulty in such cases. One is that well-marked races usually originate only in domesticated animals, or in wild animals which, owing to accidental circumstances, are placed in abnormal circumstances. Another is, that there always remains a tendency to return, in favorable circumstances, to the original type. This tendency to reversion is much underrated by Darwin and his followers; yet they constantly recur to it as a means of proving possible derivation, and their writings abound in examples of it. Perhaps the most remarkable of these reversions are those which occur when varieties destitute of all the markings of the original stock are crossed and reproduce those markings, which Darwin shows to occur in pigeons and domestic fowls. The domesticated races usually require a certain amount of care to preserve them in a state of purity, both on this account and on account of the readiness with which they intermix with other varieties of the same species. Many very interesting facts in illustration of these points might be adduced. The domesticated hog differs in many important characters from the wild boar. In South America and the West Indies it has returned, in three centuries or less, to its original form. [162] The horse is probably not known in a state originally wild, but it has run wild in America and in Siberia. In the prairies of North America, according to Catlin [163] they still show great varieties of color. The same is the case in Sable Island, off the coast of Nova Scotia [164] where herds of wild horses have existed since an early period in the settlement of America. In South America and Siberia they have assumed a uniform chestnut or bay color. In the plains of Western America they retain the dimensions and vigor of the better breeds of domesticated horses. In Sable Island they have already degenerated to the level of Highland ponies; but in all countries where they have run wild, the elongated and arched head, high shoulders, straight back, and other structural characters probably of the original wild horse, have appeared. We also learn from such instances that, while races among domesticated animals may appear suddenly, they revert to the original type, when unmixed, comparatively slowly; and this especially when the variation is in the nature of degeneracy.
4. Some characters are more subject to variation than others. In the higher animals variation takes place very readily in the color and texture of the skin and its appendages. This, from its direct relation to the external world, and ready sympathy with the condition of the digestive organs, might be expected to take the lead. In those domesticated animals which are little liable to vary in other respects, as the cat and duck, the color very readily changes. Next may be placed the stature and external proportions, and the form of such appendages as the external ear and tail. All these characters are very variable in domestic animals. Next we may place the form of the skull, which, though little variable in the wild state, is nearly always changed by domestication. Psychological functions, as the so-called instincts of animals, are also very liable to change, and to have these changes perpetuated in races. Very remarkable instances of this have been collected by Sir C. Lyell [165] and Dr. Prichard. Lastly, important physiological characters, as the period of gestation, etc., and the structure of the internal organs connected with the functions of nutrition, respiration, etc., are little liable to change, and remain unaffected by the most extreme variations in other points; and it is, no doubt, in these more essential and internal parts that the tendency survives to return under favorable circumstances to the original type.
5. Varieties or races of the same species are fully reproductive with each other, which is not the case with true species. Mutual sterility of varieties of the same species is an exceptional peculiarity, if it ever truly exist; and, on the other hand, the cross-fertilization of varieties of the same species, whether in animals or plants, tends to vigorous life, and also to return to the primitive or average type. On the other hand, intermixture of distinct species rarely, if ever, occurs freely in nature. It is generally a result of artificial contrivance. Again, hybrids produced from species known to be distinct are either wholly barren, or barren inter se, reproducing only with one of the original stocks, and rapidly returning to it; or if ever fertile inter se, which is somewhat doubtful, rapidly run out. It has been maintained by Pallas and others, and Darwin leans to this idea, that there is still another possibility, namely, that of the perfect and continued fertility of such mixed races, especially after long domestication; but their proofs are derived principally from the intermixture of the races of dogs and of poultry, which are cases actually in dispute at present, as to the original unity or diversity of the so-called species.
If we apply these considerations to man, our conclusion must be that, even in his bodily frame, he is not merely specifically but ordinally distinct from other animals, and that the differences between races of men are varietal rather than specific. This view is confirmed by the following facts:
1. The case of man is not that of a wild animal; and it presents many points of difference even from the case of the domesticated lower animals. According to the Bible history, man was originally fitted to subsist on fruits, to inhabit a temperate climate, and to be exempt from the necessity of destroying or contending with other animals. This view unquestionably accords very well with his organization. He still subsists principally on vegetable food, is most numerous in the warmer regions of the earth; and, when so subsisting in these regions, is naturally peaceful and timid. On the whole, however, his habits of life are artificial—more so than those of any domesticated animal. He is, therefore, in the conditions most favorable to variation. Again, man possesses more than merely animal instincts. His mental powers permit him to devise means of locomotion, of protection, of subsistence, far superior to those of any mere animal; and his dominant will, insatiable in its desires, bends the bodily frame to uses and exposes it to external influences more various than any inferior animal can dream of. Man is also more educable and plastic in his constitution than other animals, owing both to his being less hemmed in by unchanging instincts, and to his physical frame being less restricted in its adaptations. If a single species, he is also more widely distributed than any other; and there are even single races which exceed in their extent of distribution nearly all the inferior animals. Nor is there anything in his structure specially to limit him to plains, or hills, or forests, or coasts, or inland regions. All the causes which we can suppose likely to produce variation thus meet in man, who is himself the producer of most of the distinct races that we observe in the lower animals. If, therefore, we condescend to compare man with these creatures, it must be under protest that what we learn from them must be understood with reference to his greater capabilities.