Still, 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.
As Prof. Bergen has well said, "it is important that we should understand that none of the kinds of evidence in favor of evolution loses so much by being represented only by scattered instances as the argument from distribution." And, conversely, no argument is so strong 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 by whose eyes multitudes of species have been seen to change. To the ordinary observer the species seems 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.
And now we may notice that it was precisely this phase of the subject, the relation of species to geography, which first attracted the attention of both Mr. Darwin and Mr. 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 varies 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.
In the voyage of the Beagle, you remember, Mr. Darwin was brought in contact with the singular fauna of the Galapagos Islands, that cluster of volcanic rocks which lies in the open sea some six hundred miles west of the coast of Equador 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