220 THE BOOK OF NATURE STUDY 



countries may have quite different faunas ; from arctic America 

 to the Equator, to see that regions with very distinctive tenantry 

 are often connected by transition areas. A study of the Mammals 

 of Australia will illustrate the fact that a region may have a very 

 sharply defined fauna, and the story of the tapirs or of the camel 

 family will illustrate " discontinuous distribution/' i.e. that 

 examples of a type once widespread may survive in far separated 

 regions though there are no living representatives in the 

 intermediate areas. If these five sets of facts, for instance, 

 are presented with anything like vividness, not in this condensed 

 text-book fashion, but in narrative and picture, a grasp of some 

 of the fundamental facts as to the distribution of animals will 

 be secured almost unconsciously. 



Very gradually and cautiously, with a few good instances to 

 clinch each conclusion, the senior pupils may be led to see that 

 there are many factors which co-operate to determine why some 

 animals are here and not there, and others there and not here, 

 which is the main problem of the study of distribution. Why 

 does the Great Salt Lake of Utah contain only two or three kinds 

 of animals, instead of the dense population usually found in 

 lakes ? Why are Amphibians absent from oceanic islands ? 

 Why are small rodents so cosmopolitan ? Why are there no 

 Mammals higher than Marsupials indigenous in Australia ? W T hy 

 are there so many common features between the fauna of North 

 America and the fauna of the north of Europe and Asia ? By 

 these and a hundred similar questions the student may be led 

 to the conclusion that six main factors have contributed to the 

 present distribution of animals. They may be grouped in 

 pairs, the physical peculiarities of particular regions and the 

 constitutional peculiarities of particular animals ; the original 

 headquarters of the stock (often very uncertain) and the means 

 of dispersal in each case ; the physical changes of climate, earth 

 movements, etc. ; in particular regions and the changes brought 

 about in the struggle for existence between the various animals 

 tenanting these regions. Nor can man's influence be forgotten ; 

 for although he has not lived long upon the earth compared with 

 most other living creatures, he has been the direct cause of 

 enormous changes in distribution ; such as the introduction of 



