AA C AA FIG. 14.-A, hybrid larva of Sphærechinus, and Echinus &; front view. B, the same; oblique, side view. C, dwarf hybrid larva of Sphærechinus (non-nucleated egg fragment), and Échinus 8, of pure Echinus type. D, the same larva in side view. (After Boveri.) acteristics can be seen. The development of this larva has hence been determined solely by the nucleus of the spermatozoan-i. e., by the chromatin, which is thus experimentally demonstrated to be the bearer of the hereditary qualities in the cell, the material basis of heredity. VII. THE DISTRIBUTION OF SPECIES. Illustrations not arguments. IN the present paper I shall consider certain facts of animal distribution as related to the origin of species. There are many difficulties in bringing the facts of geographical distribution down to the needs of concrete illustration. And in this connection it is especially important to distinguish single illustrations from arguments. Isolated cases of geographical variations in species, for example, would not have great value as arguments for the development theory were the cases really isolated. The force lies in this fact, that these cases are typical; that what may be said of one is true of a thousand. In like manner the full force of the laws of homology and heredity can only be felt when their effect is cumulative, as in the mind of the anatomist who has followed each organ through its protean disguises in a wide range of forms. Again, the force of the argument drawn from embryology does not come from a knowledge of the changes in a single egg. All these studies need the second premise, obtained by years of comparison in different fields of investigation, that no case is isolated. Without this premise, the argument would be incomplete. The few cases of development or change which can be brought to popular notice are simply illustrations and not proofs. Cumulative evidence. As Professor Bergen has well said, "It is important that we should understand that none of the kinds of evidence in favour of evolution loses so much by being represented only by scattered instances as the argument from distribution." And, conversely, no argument is more conclusive when all the known facts are brought into consideration together. The universal fact of the mutability of species can be really understood or appreciated only by him who has seen with his own eyes the changes in multitudes of species. To the ordinary observer the species seem constant, just as the face of a cliff seems constant. To the student of Nature, mutability is everywhere. Just as the wind and rain and frost quietly but surely change the face of a cliff, so do other forces of Nature as quietly but as surely change the face of a species. It was this phase of the subject, the relation of species to geography, which first attracted the attention. both of Darwin and Wallace. Both these observers noticed that island life is neither strictly like nor unlike the life of the nearest land, and that the degree of difference differs with the degree of isolation. Both were led from this fact to the theory of derivation, and to lay the greatest stress on the progressive modification resulting from the struggle for existence. The fauna of the In the voyage of the Beagle Darwin was brought in contact with the singular fauna of the Galapagos Islands, that cluster of volcanic rocks which lies in the open sea about six hundred miles west of the coasts of Ecuador and Peru. The sea birds of these islands are essentially the same as those of the coast of Peru. So with most of the fishes. We can see how this might well be, for both sea birds and fishes can readily pass from the one region to the other. But the land birds, as well as the reptiles, insects, and plants, are mostly peculiar to the islands. The same species are found nowhere else. But other species very much like them in all respects are found, and these all live along the coast of Peru. In the Galapagos Islands, according to Darwin's notes, "there are twenty-six land birds; of these, twenty-one, or perhaps twenty-three, are ranked as distinct species, and would commonly be assumed to have been here created; yet the close affinity of most of these birds to American species is manifest in every character, in their habits, gestures, and tones of voice. So it is with the other animals and with a large proportion of the plants. . . . The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, feels that he is standing on American land." This question naturally arises: If these species have been created as we find them on the Galapagos, why is it that they should all be very similar in type to other animals, living under wholly different conditions, but on a coast not far away? And, again, why are the animals and plants of another cluster of volcanic islandsthe Cape Verde Islands-similarly related to those of the neighbouring coast of Africa, and wholly unlike those of the Galapagos? If the animals were created to match their conditions of life, then those of the Galapagos should be like those of Cape Verde, the two archipelagoes being extremely alike in soil, climate, and physical surroundings. If the species on the islands are products of separate acts of creation, what is there in the nearness of the coasts of Africa or Peru to influence the act of creation so as to cause the island species to be, as it were, echoes of those on shore? If, on the other hand, we should adopt the obvious suggestion that both these clusters of islands have been |