142 ZOOLOGY 



semblance between the native products. It is an ob- 

 vious suggestion that some of the islands were more 

 recently connected than others, and that evolution has 

 been going on since they became separated. Such 

 theories are beautifully illustrated by the animals of the 

 Hawaiian Islands, and especially the remarkable snails 

 (Achatinellida) characteristic of the group. 



The exceptions to the principles just cited are quite 

 numerous, but they can often be explained, and it is 

 presumed that only our ignorance prevents the explana- 

 tion of all. For example, the llama of South America 

 belongs to the Camelidae or camel family. That it 

 should exist so far away from its relatives, the camels, 

 seems quite anomalous, and contrary to the idea of 

 descent from a common ancestor. The explanation is 

 found in the presence of great numbers of camel fossils 

 in North America ; the camel group once extended all 

 over the western hemisphere, as well as over Asia, but 

 has now left only remnants at the ends of its range. 

 The opossum, a marsupial, is far removed from its 

 marsupial relatives in Australia; but we know that 

 marsupials once existed in every continent and in great 

 variety. The development of the higher mammals 

 crowded them to the wall, and they now survive in a 

 very small part of their former territory. 



Changes in 8. Darwin was influenced by the well-known fact 

 piantsfunder that many domesticated animals and cultivated plants 

 the com- have changed greatly under the influence of man. The 



paratively 11111. i 11 i 



brief in- race horse and the dahlia, the pig and the plum, are no 

 fluenceof longer what they were a few centuries ago. Man has 

 chosen what he wanted from among the variations 

 afforded by Nature, and has preserved and propagated 

 many beautiful and useful types. He has, when the 

 fancy took him, developed the grotesque or even 



man 



