descent that at best can only be interrupted 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 re- 

 sults 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 

 Buff on 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 knowl- 

 edge of animal distribution. From this many well-estab- 

 lished 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 sup- 

 posed. As a statement of the results in this apparently 

 simple, but really quite complicated field would be mislead- 

 ing, I fear, from its brevity and general form, I will ven- 

 ture to present just one conclusion. Geographical isola- 

 tion 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 con- 

 nected 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 con- 

 tains the only egg-laying mammals known, as well as all of 

 the pouched mammals like the kangaroo, with a few ex- 

 ceptions like our American opossum. Furthermore, groups 



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