unfounded; that the very same causes which are at work to-day, such as rain, wind, heat, frost, and imperceptible risings and sinkings of the earth's surface, acting for millions of years, are sufficient to account for the present formation of the earth's crust, and, without colossal volcanic eruptions, to raise the highest chains of mountains.

Lyell's work entirely destroyed the theory of catastrophes, and yet it continued to be assumed that the whole plant and animal world was recreated at the beginning of each new period of the earth's history. It was the great English naturalist Charles Darwin who succeeded in establishing the truth that the plants and animals of to-day have arisen from lower living forms by such slow and continual changes as have produced the present various features of the earth's crust from earlier and different ones. It was not only that when he wrote the times were ripe and men's minds prepared for the acceptance of the theory of evolution. Darwin, besides treating the views put forth by Lamarck and others much more fully and clearly than they had done, gave in his doctrine of natural selection an explanation of the changes in living things, whereas his predecessors had merely stated them as facts. The reasoning on which he based his theory is not universally accepted at the present day, but it brought home to men's minds, as nothing before had done, conclusions which are everywhere acknowledged as true.

It was in 1858 that Darwin published his Theory of Natural Selection, which was carried out in greater detail in The Origin of Species, 1859. Spencer, Huxley, Haeckel, and many others had been working on the same lines, and Alfred Russel Wallace reached the same conclusions as Darwin on natural selection and

published a treatise on it at the same time. The Origin of Species was the result of twenty years' labour, from Darwin's voyage as naturalist in the Beagle (1831-1836) to the date of publication. The following extract from the introduction is worth quoting for the light it throws on scientific method:

"When on board H.M.S. Beagle as naturalist, I was much struck with certain facts in the distribution of the organic beings inhabiting South America, and in the geological relations of the present to the past inhabitants of that continent. These facts seemed to throw some light on the origin of species-that mystery of mysteries, as it has been called by one of our greatest philosophers. On my return home, it occurred to me, in 1837, that something might perhaps be made out on this question by patiently accumulating and reflecting on all sort of facts which could possibly have any bearing on it. After five years' work I allowed myself to speculate on the subject, and drew up some short notes; these I enlarged in 1844 into a sketch of the conclusions, which then seemed to me probable: from that period to the present day I have steadily pursued the same object. . . . My work is now (1859) nearly finished; but as it will take me many more years to complete it, and as my health is far from strong, I have been urged to publish this Abstract."

It was, then, what he saw in South America and in the Galapagos Islands, 600 miles distant from the west coast, that caused Darwin to conceive that new theory of the origin of species which he spent so many years in patiently testing and working out. The view of species commonly held at the time was that each had been separately created. Darwin maintained that many of the so-called species were merely variations of an older

common stock, that in remote epochs these stocks were fewer and resembled each other more nearly, and that the failure in many cases to trace the intermediate links between existing and extinct types, or to verify the existence of a parent stock from which types strikingly diverse in appearance had nevertheless sprung, was due not to their non-existence, but to their having for various reasons failed to survive in fossil form. The causes of variation he maintained to be not only external conditions such as climate, food, etc., but the working of natural selection, those individuals of a species and those species in general having the best chance of surviving which were best fitted to their environment.

During his visits to the different islands of the Galapagos group, now divided from each other by deep arms of the sea, and 600 miles distant from the mainland, though perhaps formerly connected with it and each other, Darwin was much struck by the fact that each island had plants and animals, especially reptiles and birds, which, in spite of the special facilities of the latter for migrating from one island to another, were peculiar to it, though in many cases species were common to more than one island, and nearly every island had species in common with the neighbouring American mainland. Moreover, the closer the islands were to one another the more resemblance there was in their plants and animals. This was unintelligible unless it meant that corresponding species on different islands had a common descent, and that the differences had been produced by changes in environment.

Again, by comparing the living animals of the continent with fossil remains, he found a striking resemblance between the existing animals and the extinct,

the conclusion being that they were blood relations from a common stock.

sprung

Armed with these facts, Darwin came home in 1837 to collect all the facts he could find bearing on the theory he had formed of the origin of species by natural selection, to weigh the evidence, and to draw up his conclusions.

He first carefully studied instances of variation in domesticated animals and plants. He gave great attention to domestic pigeons, and came to the conclusion that in spite of the wonderful diversity of the different breeds-carrier, tumbler, runt, barb, pouter, fantail, turbit, Jacobin, etc.--they were all descended from the rock pigeon, and that their peculiarities were produced by the deliberate adding up of variations given by nature—that is, by the mating of a bird displaying a certain peculiarity with another possessing the same. He considered what was being done every day to produce rapid variation in the breeding of horses, cattle, and sheep, and in the cultivation of plants. The greater variability of domesticated animals and plants compared with that of the same in the natural state he decided to be due to changes in the conditions of life and to more abundant food.

Turning to variation under nature, Darwin found its origin in the struggle for existence. Variations, he says, if they be in any degree profitable to the individuals of a species, will tend to the preservation of such individuals and will generally be inherited by their offspring, which in their turn will have a better chance of survival. This principle, to mark its relation to man's power of selection, Darwin called natural selection; Herbert Spencer described it by the expression, the 66 survival of the fittest." The struggle for existence

is due to the fact that all living beings tend to increase at a higher rate than the means of life will support. In nature, therefore, there must be great destruction and waste. Out of a hundred seedlings produced by a plant only one may reach maturity, because they must all compete with other plants for light and air and the food contained in the soil. Again, each new variety or species that is undergoing improvement presses hard on its unimproved kindred and tends to exterminate them because it is better equipped for the struggle to obtain food. To take a comparison from artificial selection, or breeding: improved breeds of cattle, sheep, and other animals, and better varieties of plants take the place of the older and inferior kinds. In Yorkshire the ancient black cattle were supplanted by longhorns, and, says a writer, “these, again, were swept away by shorthorns as if by a murderous pestilence." "When we reflect on this struggle," says Darwin, " we may console ourselves with the full belief, that the war of nature is not incessant, that no fear is felt, that death is generally prompt, and that the vigorous, the healthy, and the happy survive and multiply."

Again, we find adaptations of structure as the result of use or disuse. The domestic duck has a bigger leg bone and a smaller wing bone than the wild duck, because it walks more and flies less. The eyes of moles and other burrowing animals are rudimentary in size and sometimes quite covered with skin and fur. Animals inhabiting the depths of caves, such as the Mammoth Cave in Kentucky, are often blind, or even devoid of eyes. Those living near the mouth of the cave have sight suited to a twilight existence. There is no doubt that they are descended from ancestors having normal vision and losing it as successive