CH. XII GEOGRAPHICAL DISTRIBUTION OF ORGANISMS 339 



familiar. He would also anticipate that diversities of climate 

 would always be associated with a corresponding diversity in 

 the forms of life. 



Now these anticipations are to a considerable extent justi- 

 fied. Remoteness on the earth's surface is usually an indi- 

 cation of diversity in the fauna and flora, while strongly 

 contrasted climates are always accompanied by a considerable 

 contrast in the forms of life. But this correspondence is by 

 no means exact or proportionate, and the converse propositions 

 are often quite untrue. Countries which are near to each 

 other often differ radically in their animal and vegetable pro- 

 ductions ; while similarity of climate, together with moderate 

 geographical proximity, are often accompanied by marked 

 diversities in the prevailing forms of life. Again, while many 

 groups of animals — genera, families, and sometimes even 

 orders — are confined to limited regions, most of the families, 

 many genera, and even some species are found in every part 

 of the earth. An enumeration of a few of these anomalies ^vill 

 better illustrate the nature of the problem we have to solve. 



As examples of extreme diversity, notwithstanding geo- 

 graphical proximity, we may adduce Madagascar and Africa, 

 whose animal and vegetable productions are far less alike than 

 are those of Great Britain and Japan at the remotest ex- 

 tremities of the great northern continent ; while an ecjual, or 

 perhaps even a still greater, diversity exists between Australia 

 and New Zealand. On the other hand. Northern Africa and 

 South Europe, though separated by the Mediterranean Sea, 

 have faunas and floras which do not differ from each other 

 more than do the various countries of Europe. As a proof 

 that similarity of climate and general adaptability have had 

 but a small part in determining the forms of life in each 

 country, we have the fact of the enormous increase of rabbits 

 and pigs in Australia and New Zealand, of horses and cattle 

 in South America, and of the common sparrow in North 

 America, though in none of these cases are the animals 

 natives of the countries in which they thrive so well. 

 And lastly, in illustration of the fact that allied forms are 

 not always found in adjacent regions, we have the tapirs, 

 which are found only on opposite sides of the globe, in 

 tropical America and the Malayan Islands ; the camels of 



