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which the theory took in the hands of its earlier propounders, but only that which it has assumed since it was treated by Darwin and Wallace. These naturalists -particularly Mr Darwin-have so transformed it as to make it almost a new thing, and have supported their views by so many facts and arguments, that they are meeting with very wide acceptance both in this country and abroad.
The Theory of the Evolution of Living Things has for its main object to show that all the phenomena of living things-all their wonderful organs and complicated structures, their infinite variety of form, size, and colour, and their instincts and involved relations to each other— may have been produced by the action of a few general laws of the simplest kind,-laws which are in most cases mere statements of admitted facts. It will be our business now to state and illustrate these laws.
§1. "Natural Selection."
The Origin of Species by Means of Natural Selection is the title of Mr Darwin's first book on this subject; but it is only by giving an enlarged sense to the words that they can be made to include the whole of his theory. It is in the larger sense that they are used at the head of this section.
The Law of Heredity.-Starting with the fact of life, and the great abundance of living forms, we observe that the individuals are subject to death, but that commonly they leave behind them offspring possessing the same form and characters, the likeness frequently extending to even the smallest individual peculiarities. Why like
1 Where not otherwise stated in the notes, the facts of this section will be drawn from the works of Messrs Darwin and Wallace.
VARIATION FROM PARENTAL TYPE.
should produce like we do not know; but although the fact is as much a marvel as it would be for parents and offspring to be always widely dissimilar, we have come to regard it as the natural, and therefore the less wonderful course.
The Law of Variation.-If the first great fact regarding reproduction is that the offspring resemble the parents, it is a fact equally undeniable and remarkable that the resemblance is never complete. Children of the same parents are not all alike among themselves, nor exactly like either their immediate parents or remote ancestors, though they commonly resemble them more than they resemble strangers. The deviation from the parental type is not in one particular only, but the copies slightly differ from the original and from one another in every possible way-in form, in size, in complexion; in the structure of internal as well as of external organs; in those subtle peculiarities which produce differences of constitution, and in those which lead to modifications of mind and character.
This fact of variation is equally true of man, of all animals, and of all plants. Looking at our domestic animals, horses of the same parentage will differ in colour, size, strength, and speed; rabbits will be grey, brown, white, black, &c.; pigeons differ in size, colour, and habits-in the size and shape of the bill and the feet, the number of feathers in the tail, &c. With dogs and cats, the same law of variation holds true, as well as with sheep, and oxen, and poultry. Wild animals would supply us with fresh instances; as would also the vegetable world, whether we looked at wild or cultivated species, at fruits or flowers-there is absolutely no exception.
The Correlation of Variations.-Variations are frequently correlated or linked together in such a way that when one appears a second appears with it—a sort of Siamese twin companion inseparable from its brother. A sixth finger on one hand is usually accompanied by a supernumerary digit on the other; when the muscles of the arms depart in number or arrangement from their proper type, they do so in company, and almost always imitate those of the leg. It is observed that when white. cats have blue eyes, they are generally deaf also; when pigeons have short beaks, they have also small feet; the naked Turkish dog has imperfect teeth, and this relation between hair and teeth seems to be general. With man several striking cases have been recorded of inherited baldness with inherited deficiency of the teeth, either complete or partial; and in those rare cases in which the hair has been renewed in old age, there has usually been a renewal of the teeth. Mr Crawford saw, at the Burmese Court, a man, thirty years old, with his whole body, except the hands and feet, covered with straight silky hair, which, on the shoulders and spine, was five inches in length, though at birth the ears alone were covered. This man did not arrive at puberty nor shed his milk teeth until twenty years old, and at this period only acquired the eight incisor teeth and one canine. Another case is that of Julia Pastrana, a Spanish dancer, who was a remarkably fine woman, but possessed a thick masculine beard and a hairy forehead, peculiarities which were accompanied by an irregular double set of teeth-one row within the other-both in the upper and the lower jaw. Turning for a moment to plants, we observe that in double flowers the stamens and pistils vary in the same manner, and assume the form
and colour of the petals. In some cases the flowers and leaves vary together in tint; in all the varieties of the common pea which have purple flowers, a purple mark appears also on the stipules.
The Transmission of Variations.—The offspring, then, inheriting in general the characters and qualities of the parents, yet differs from the parents in many little ways; and now when it comes to produce offspring in its turn, it will tend to transmit its own (slightly altered) characters, and, owing to the law of variation, will not succeed perfectly even in that. Moreover, observation shows that peculiarities may be transmitted to the offspring of both sexes alike, or to one sex alone, and may show themselves at birth or at various later periods of life. In most species of parrots, both sexes are brilliantly coloured and undistinguishable. In woodpeckers also the two sexes are generally nearly alike. The equal transmission of characters to both sexes is the commonest form of inheritance. But characters are not rarely transmitted exclusively to that sex in which they first appeared. There are breeds of the sheep and goat in which the horns of the male differ greatly in shape from those of the female, and these differences, acquired under domestication, are regularly transmitted to the same sex. With tortoise-shell cats, the females alone, as a general rule, are thus coloured, the males being rusty-red. The long and magnificent train of the peacock contrasts with the short tail of the pea-hen, each sex inheriting its own. In the Spanish fowl the male has an immense comb; and while the comb of the female is also large, there is this difference, that the comb of the male is upright, but that of the female is apt to lop over. Variations occurring late in life tend
to be transmitted exclusively to the same sex; whilst variations which first appear early in life, in either sex, tend to be developed in both sexes. If a new character appears in an animal whilst young, whether it endures throughout life or lasts only for a time, it will reappear, as a general rule, at the same age and in the same manner in the offspring. If, on the other hand, a new character appears at maturity, or even during old age, it tends to reappear in the offspring at the same advanced age. The pigeon offers a remarkable instance of this; there are breeds which do not acquire their characteristic colours until they have moulted two, three, or four times, and these modifications of plumage are regularly transmitted. With animals in a state of nature, innumerable instances occur of characters periodically appearing at different seasons, as, e.g., the fur of Arctic animals becomes thick and white during the winter; and numerous birds acquire bright colours and other decorations during the breeding season alone. Another remarkable fact is, that characters, instead of simply lying dormant in the individual for a few months or years, and appearing at a certain stage of growth, may be transmitted through several generations without showing themselves, and then make their appearance in the descendants. Every one knows that insanity and consumption run in families, as also does the speed of the racehorse; and that such diseases and qualities will sometimes pass over the child and reappear in the grandchild. When the individual reproduces the characters of some remote ancestor, instead of those of its immediate parents-as when the Suffolk cattle, which have been hornless for more than a century, produce a horned calf-this is called a case of reversion.