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Secondly, by Sexual Selection. The modifications acquired through this agency are often so strongly pronounced, that the two sexes have frequently been ranked as distinct species, or even as distinct genera. Two humming birds belonging to the genus Eustephanus, which inhabit the island of Juan Fernandez, were long thought to be specifically distinct, but are now known, Mr Gould says, to be the sexes of the same species. Among butterflies, the females of Papilio memnon and Diadema auge consist of scores of such different insects, that they have over and over again been described as distinct species, but in both cases the males are very constant. Bearing these facts in mind, and remembering the marked results of man's unconscious selection, it seems certain that if the individuals of one sex were during a long series of generations to prefer pairing with certain individuals of the other sex, characterised in some peculiar manner, the offspring would slowly but surely become modified in accordance, and it is difficult to assign any limit to the amount of change.
Thirdly, since man has produced a great result by his methodical and unconscious selection, what may not Natural Selection effect? Man can only act on external and visible characters; nature acts on every internal organ, on every shade of constitutional difference, on the whole machinery of life. Man selects only for his own good; nature only for that of the being which she tends. Every selected character is fully exercised when nature alone is superintending, and the being is placed
offered them, and those that show any slight preference for one kind above another be set apart and nourished as exclusively as possible on that kind, and the process repeated with their offspring.
under well-suited conditions of life. Man keeps the natives of many climates in the same country, exposes sheep with long and short wool to the same conditions, feeds a long and a short beaked pigeon on the same food. He does not exercise a long-backed or longlegged quadruped in the peculiar manner its structure. demands; he does not allow the most vigorous males to struggle for the females; he does not destroy all inferior individuals. He begins his experiments, and dies while they are in progress; but nature never dies, nor sleeps, nor forgets to work, silently and insensibly improving each organic being in relation to its conditions of life. We observe nothing of these slow changes while in progress-slow like the minute-hand of a clock; but when the hand of time has marked the long lapse of ages, we see that the forms of life have become different from what they formerly were. How fleeting are the wishes and efforts of man! how short his time! and consequently how poor will his products be compared with those accumulated by nature during long geological periods !
The Origin of Man.-Man is an animal built up on the same plan as other animals of the class Mammalia, and all the bones in his skeleton can be compared with corresponding bones in a monkey, a bat, or a seal. So it is for the most part with his muscles, nerves, bloodvessels, and internal viscera; while every chief fissure and fold of his brain has its analogy in that of the orang. Many modern naturalists, following Linnæus, have placed him in the same order with the Quadrumana; and Professor Huxley declares that the structural differences which separate man from the gorilla and chimpanzee are not so great as those which separate the
gorilla from the lower apes.1 Man passes through the same phases of embryological development with other vertebrates, so that at an early stage his embryo can hardly be distinguished from theirs. Many muscles regularly present in apes and other quadrupeds appear in man occasionally, and numerous rudiments occur of structures characteristic of lower forms, but altogether useless in him. The internal and external parasites of man and the lower animals are of the same families and genera, and he is able to receive some animal diseases (as glanders and hydrophobia), showing a close similarity in blood and tissues. But if man be separated by no greater structural barrier from the brutes than they are from one another, then it seems to follow that if any process of physical causation can be discovered by which the genera and families of ordinary animals have been produced, that process is amply sufficient to account for the origin of man.2
A multitude of facts prove that man is liable to numerous slight and diversified variations, which are induced by the same general causes, and are governed and transmitted in accordance with the same general laws, as in the lower animals. He tends to increase at a greater rate than his means of subsistence, and consequently is occasionally subjected to a severe struggle for existence, combating with the elements, with the brute creation, and with men of other tribes; so that natural selection comes into play, and of course effects whatever lies within its scope.
From a thorough consideration of these facts, and others allied to them, Mr Darwin infers that man is de
1 Huxley Man's Place in Nature, p. 103.
EVOLUTION WIDER THAN SELECTION.
scended from a hairy quadruped, furnished with a tail and pointed ears, probably arboreal in its habits, and an inhabitant of the Old World. This creature, if its whole structure had been examined by a naturalist, would have been classed amongst the Quadrumana, as surely as would the common and still more ancient progenitor of the Old and New World monkeys.
§ 2. The Cause of Variations.
The gradual lapse of time has now separated us by more than a decade from the date of the Origin of Species; and whatever may be thought or said about Mr Darwin's doctrines, or the manner in which he has propounded them, this much is certain, that in a dozen years the Origin of Species has worked as complete a revolution in biological science as the Principia did in astronomy; and it has done so because, in the words of Helmholtz, it contains "an essentially new creative thought."1. But the theory of Natural Selection is not a complete theory of the evolution of living things. Natural Selection accounts for the preservation and accumulation of useful variations, till new species are thereby evolved; Evolution seeks in addition to show the causes of the variations themselves, and the manner of origin of living things in the first instance. Natural Selection is a part, Evolution is the whole, and the part falls into its right place when the whole scheme is expanded.
One of the most frequent objections made to the Darwinian theory has been, that it does not account for the origin of the "variations" which are afterwards "selected," and that unknown laws or tendencies must be at work within the organism, or without. It has 1 Professor Huxley: Contemp. Rev., Nov. 1871.
been said that Natural Selection manifestly does not produce the variations, nor does it even suggest the law under which, or by which, or according to which, new forms are introduced.1 Natural Selection has nothing to do with the creation of any favourable addition to nature; it is only the removal of those who do not possess the addition. The facts suggest to the mind the idea of some creative law, almost as certainly as they convince us that we know nothing of its nature, or of the conditions under which it does its glorious work. It seems probable that new species may arise from some constitutional affection of parental forms—an affection mainly, if not exclusively, of their generative system. It is not only possible, but highly probable, that an internal power or tendency is an important, if not the main, agent in producing the manifestation of new species on the scene of realised existence.5
Mr Darwin is so fully aware that his theory does not account for the initial variations, that he over and over again ascribes them to "unknown causes; and the objections are so far from being valid against Natural Selection that they only ask to have the doctrine supplemented. This supplement to Natural Selection-or rather this complement, for the two together constitute a tolerably complete theory-is supplied in Mr Herbert Spencer's teaching, that the changes in an organism are produced by the changing conditions of its environment, which are traceable ultimately to the laws of matter and
1 Reign of Law, by the Duke of Argyll, 2d ed., pp. 230, 255. 2 Quart. Rev., July 1869, p. 162.
3 Reign of Law, p. 249.
4 Mr St George Mivart: Genesis of Species, 2d ed., p. 263. 5' Ibid., p. 278.