372 ANIMAL BIOLOGY 



each other having species most similar. Darwin wrote, " My atten- 

 tion was first thoroughly aroused by comparing together the 

 numerous specimens, shot by myself and several others on board, 

 of Mocking Thrushes, when, to my astonishment, I discovered 

 that all those from Charles Island belonged to one species 

 (Mimus trijasciatus) ; all from Albemarle Island to M. parvulus; 

 and all from James and Chatham Islands (between which 

 two other islands are situated as connecting links) belonged to 

 M. melanotis." 



Darwin's observations of such facts as these have been cor- 

 roborated in the Galapagos and extended to isolated island faunas 

 and floras all over the world. For instance, half of the species of 

 Insects and four-fifths of the species of Seed Plants that occur on 

 St. Helena are found nowhere else. And further, Darwin's ex- 

 planation of the phenomena is the most plausible extant. Conti- 

 nental islands secure their life from the mainland before they are 

 cut off, and oceanic islands after their formation by volcanic action 

 alone or aided by coral growth. In either event the organisms in- 

 habiting islands are isolated from the main stock of the species, 

 and they diverge, in proportion to the length of time and the degree 

 of isolation, until they constitute separate races and species. 

 Isolation promotes divergence apparently to a considerable extent 

 by preventing new types from being swamped by interbreeding 

 with the old, and by allowing many mutations to become established 

 that would not survive in a more competitive field. Furthermore, 

 new conditions may afford new problems to be met by mutations, 

 and so favor their persistence. We see evolution as a response of 

 life to its environment. Each species peculiar to each isolated 

 island can reasonably be interpreted as having arisen by descent 

 with change. (Figs. 236-238.) 



We have now summarized a few concrete examples of the chief 

 types of evidence that organisms — species — have come to be 

 what they are to-day through a long process of descent with change. 

 This evidence, taken with that presented, so to speak, on and be- 

 tween the lines throughout this work, should place the reader in a 

 position to form a more or less independent judgment of the ques- 

 tion. It is only necessary to remind him again that, although the 

 evidence, from the nature of the case, must inevitably be indirect, 

 its force is tremendously increased by its amount. And the reader, 

 with only a very limited amount of the data before him, cannot 



