ZOOLOGY 447 



fill in the gaps of the lines of descent that at best can only be inter- 

 rupted lines, and to show how these lines lead to modern forms or to 

 divergent kinds that have ceased to be. And he will compare his results 

 with those of students in other fields, who will assist him to formulate 

 the working-plans for his own labors. 



Zoo-geography is the last branch of structural zoology to attain an 

 independent status. Many observers from Buffon onward had been 

 struck by the fact that species of animals are not uniformly distributed 

 over the earth, that they differ more widely as the observer passes to 

 more and more remote localities, with more different climatic and other 

 environmental conditions. But the meaning of these peculiarities was 

 obscure until the doctrine of descent cleared their vision. Wagner, 

 Louis Agassiz and Dana, Sclater, Murray and Wallace were the leaders 

 of those who have brought together the immense mass of modern 

 knowledge of animal distribution. From this many well-established 

 principles relating to descent have been derived, though these have a 

 deeper interest in connection with the dynamic problem as to whether 

 differences in environment can actually cause species to transform, as 

 Lamarck supposed. As a statement of the results in this apparently 

 simple, but really quite complicated field would be misleading, I fear, 

 from its brevity and general form, I will venture to present just one 

 conclusion. Geographical isolation corresponds in a general way with 

 the divergence of species in their evolution from common ancestors; 

 thus widely separated areas have faunas that differ more widely in 

 zoological respects than do those of neighboring or connected countries. 

 For example, the Australian region has been cut off for a relatively long 

 period from neighboring continents, and in correspondence with this 

 isolation it contains the only egg-laying mammals known, as well as 

 all of the pouched mammals like the kangaroo, with a few exceptions 

 like our American opossum. Furthermore, groups of isolated oceanic 

 islands, like the Galapagos and Azores and the clusters of Polynesia, 

 are inhabited by lizards and birds and insects which resemble most 

 closely the species of the nearest bodies of land. Such resemblances are 

 most reasonably interpreted as indicating that the original progenitors 

 of the island colonies were stragglers from the nearest mainland, 

 whose descendants have undergone divergent evolution during succeed- 

 ing generations. 



Having, then, this vast store of fact and principle amassed through 

 centuries by countless students, the zoologist is entitled to speak posi- 

 tively when he finds a law like the doctrine of evolution that reviews 

 and summarizes the whole range of animal structure. The well- 

 established facts of zoology are the reasons why he asserts with a 

 decision often mistaken for dogmatism that evolution is a real process. 

 The further question, why is nature so constituted that evolution is 



