GEOGRAPHICAL DISTRIBUTION 311 



it at once raises a series of questions as to the reasons for each 

 of the facts in geographical distribution, for it is the duty of 

 science to suppose that none of these facts is arbitrary or mean- 

 ingless. Each fact has some good cause behind it. 



It was this phase of the subject, the relation of species to 

 geography, which first attracted the attention of both 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 varies with the degree of 

 isolation. Both were led from this fact to the theory of deriva- 

 tion, and to lay the greatest stress on the progressive modifica- 

 tion resulting from the struggle for existence. 



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 largely peculiar to the islands. 

 Many of these species are found nowhere else. But other 

 species very much like them in all respects are found, and these 

 live along the coast of Peru. In the Galapagos Islands, ac- 

 cording 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 be commonly 

 assumed to have been here created; yet the close affinity of the 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 vol- 

 canic 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 the coast not far away? 

 And, again, why are the animals and plants of another cluster 

 of volcanic islands the Cape Verde Islands similarly related 



