organic thing," says he, " has arisen out of slime, and is nothing but slime in different forms. This primitive slime originated in the sea, from inorganic matter in the course of planetary evolution." 1

About the same time another German philosopher, Treviranus, strongly advocated the theory of evolution and the mechanical origin of life. "In every living being," he remarked, "there exists the capability of endless variety of form-assumption; each possesses the power to adapt its organisation to the changes of the outer world, and it is this power put into action by the change of the universe that has raised the simple zoophytes of the primitive world to continually higher stages of organisation, and has introduced a countless variety of species into animate nature." 2

It is with surprise and admiration at his wonderful insight that we find Kant, in 1790, pointing to the theory of descent, based upon comparative anatomy, as the only legitimately scientific explanation of the agreement of animals in a certain common plan of structure. The following passage is in complete harmony with the evolutionary doctrines of the present day: "This analogy of forms (in so far as they seem to have been produced in accordance with a common prototype, notwithstanding their great variety) strengthens the supposition that they have an actual blood relationship, due to origination from a common parent; a supposition which is arrived at by observation of the gradual approximation of one class of animals to another, beginning with the one in which the principle of purposiveness seems to be most conspicuous, that is, man, and extending down to the polyps, and from these even down to mosses and lichens, and arriving finally at raw matter, the lowest stage of nature observable to us."

Not, however, till we come to Lamarck does the de

1 Häckel, History of Creation, translated by E. Ray Lancaster.

2 Ibid.

velopment theory assume a methodical or systematic form. Though born in 1744, his memorable work, the Philosophie Zoologique," was only published in 1809. Starting with spontaneous generation, he believed nature to be daily employed in creating the beginnings of life, according to the same physical law that pervades all her other works. The lowest plants and simplest animals equally sprung from these beginnings. Different surroundings produced different results. Use and disuse perpetually modify and permanently affect organisation. He supported his doctrine by an appeal to facts with which. every one is conversant. He instanced the effect upon plants and animals of varied situation and climate, of culture and of domestication. And he called geology to witness that, time was an element in the calculation which might be drawn upon without stint.

There were, unfortunately, some slight defects in Lamarck's argument, which gave his opponents a temporary advantage. He ascribed development to the operation of two principles. One was inherent tendency to progressive advancement; the other was the force of external circumstances-" par l'influence des circonstances d'habitation, et par celles des habitudes contractées." Now, although the force of circumstances is a real factor of evolution, both these principles are open to objection, inasmuch as they fail to account for a great part of the facts. To one who invoked natural causes solely, reference to an intrinsic tendency was an infraction of his own rules. In the first place, his tendency is no better than his sentiment interieur or his movements des fluides, with which he indirectly sought to explain it. As to progress, it was easy to show that many of the lowest organisms had made no advance for countless ages; that there are cases in which the mature animal is less perfect than its larva; that instances of degradation are abundant, and (if the theory of descent be true) of reversion to ancestral types. Setting aside the mystical explanation, which is itself

unintelligible, it is evident that no principle of direct adaptation alone can account for the change in forms, or (in the words of Mr. Spencer) for "all the modifications that serve to refit organisms to their environments." Taking, for example, defensive appliances: "Suppose a species of nettle bare of poison-hairs to be habitually eaten by some mammal intruding on its habitat, the agency of this mammal would have no direct tendency to develop poison-hairs in the plant, since the individuals devoured could not bequeath changes of structure, even were the actions of a kind to produce them; and hence the individuals that perpetuated themselves would be those on which the new incident force had not fallen." 1 Another apt illustration is the following: "How by any process of direct equilibration could it [an egg-shell] come to have the required thickness? or, indeed, how could it come to exist at all? Suppose this protective envelope to be too weak, so that some of the eggs a bird lays are broken or cracked. In the first place, the breakages or crackings are actions of a kind which cannot react on the maternal organism in such a way as to cause the secretion of thicker shells for the future; to suppose that they can is to suppose that the bird understands the cause of the evil, and that the secretion can be controlled by its will." 2

Great, then, as were the services rendered by Lamarck, his system was not complete; some principle had yet to be discovered to supplement the process of direct adjustment. This discovery was made almost simultaneously and quite independently by Darwin and by Mr. Wallace. It is impossible to state the doctrine of natural selection more briefly or more clearly than in Darwin's own words: "As many more individuals of each species are born than can possibly survive, and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary, however slightly, in any manner profitable to itself, under the complex and sometimes varying 1 Principles of Biology, § 161. 2 Ilid., § 162. VOL. I.

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conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form."1

In a letter to Professor Häckel in 1864, Darwin describes how the theory of selection was suggested to him by observations made during his voyage in the "Beagle" thirty years previously. He had been profoundly struck by the close affinity of species in neighbouring countries, and in islands and their nearest continents; also by the likeness of living Edentata and Rodentia to extinct species. This general resemblance indicated descent from a common parent. "But for some years," he writes, "I could not conceive how each form became so excellently adapted to its habits of life. I then began systematically to study domestic productions, and after a time saw clearly that man's selective power was the most important agent. I was prepared, from having studied the habits of animals, to appreciate the struggle for existence, and my work in geology gave me some idea of the lapse of past time. Therefore when I happened to read 'Malthus on Population,' the idea of natural selection flashed on me."

In 1857, two years before the publication of the "Origin," Darwin had written to the famous American botanist, Professor Asa Grey, giving an outline of his projected treatise on natural selection. In this letter also he referred to the tendency of human beings to increase in a much higher ratio than the means of subsistence, which Malthus had been the first to proclaim.2 For over twenty years before publishing, Darwin devoted his wonderful powers of observation, and his no less remarkable energy and sagacity, to collecting proofs for the establishment of his theory.

Some years earlier, 1855, Mr. Wallace, then at Sara

1 Origin of Specics, p. 4.

2 Malthus appears to have taken the idea from Benjamin Franklin.

See Observations Concerning the Increase of Mankind, 1757.

wak, wrote an essay "On the Law which has Regulated the Introduction of New Species." His observations on geological and geographical distribution had led him, as they had led Darwin, to adopt the theory of descent. From the facts of affinity, geographical distribution, geological sequence, and the phenomena of rudimentary organs, he deduced the following law: "Every species has come into existence coincident both in space and time with a pre-existing closely allied species." In 1858, Mr. Wallace wrote "On the Tendency of Varieties to Depart Indefinitely from the Original Type." He here pointed to the struggle for existence and to the law of population. He illustrated this law by supposing a pair of birds to produce four young ones four times only in their lives. In fifteen years the numbers would amount to more than two thousand millions. "Whereas we have no reason to believe that the number of the birds of any country increases at all in fifteen or in one hundred and fifty years." He also distinctly enunciated the principle of natural selection. Speaking of the tendency of useful variations to preserve, and of their absence to diminish, the individuals of a species, he says, "If . . . any species should produce a variety having slightly increased powers of preserving existence, that variety must inevitably in time acquire a superiority in numbers." In the same. essay, comparing his own theory with Lamarck's-that progressive changes in species have been produced by the attempts of animals to increase the development of their own organs, and thus modify their structure and habitsMr. Wallace denies that the giraffe lengthened its neck by stretching it; but explains that "any varieties which occurred among its antitypes with a longer neck than usual at once secured a fresh range of pasture, and on the first scarcity of food were thereby enabled to outlive them." Again, Mr. Wallace was the first to elaborate the marvellous principle of mimicry, the discovery of which is due to Mr. Bates.

1 Journal of the Proceedings of the Linnean Society.