CHAPTER 21 

 GEOGRAPHIC DISTRIBUTION 



The locations of species on the earth have been determined by two 

 general sets of factors, the ecological and the historical. Animals must 

 live in situations which are at least moderately favorable to them, but 

 they are able to occupy suitable areas only if these are within reach. 

 Many excellent sites are not occupied because they are far away, and 

 there is no adequate means of transport. Moreover, most animals can- 

 not be assumed to have purpose, and they cannot have knowledge of 

 the conditions of life in other places. Accordingly, if they find new 

 locations it must be as a result of normal activities, including some events 

 which must be regarded as accidental. 



The purely local distribution of species, which depends on ecological 

 factors, has been discussed in the preceding chapter. While it will be 

 necessary to point out relations to the environment in this chapter also, 

 only such relations as bear on the history of distribution will be included. 

 Let us see how animals have come to be where they are. 



Interplay of Two Evolutions. — While present distribution of living 

 things has often been used to prove that evolution has occurred, an 

 understanding of zoogeography is most easily attained by reversing the 

 arguments. If it be assumed that evolution has taken place, many 

 peculiarities of distribution have a natural explanation. 



There are two of these evolutions, independent of each other in their 

 origins, but with intert^vining results. One is the evolution of species 

 of animals and plants, the other the evolution of the earth on which 

 they live. New species have arisen out of older species, ever since life 

 began. A group of individuals becomes different from their fellows, 

 through mutation and recombination of genes and other events, and a 

 new species is started. Usually the new species finds or at any rate 

 occupies an area somewhat different from that of the other species. In 

 time it gives rise to further new species, which take up their special 

 locations. As more and more new species arise, there is a cleavage 

 among them; some of them are much more alike, but differ strikingly 

 from another group within which the species are rather similar. Genera 

 are thus produced. As species change still more and more, there is 

 cleavage among the genera, and families arise. Continued change of 

 species results in divisions of higher ranks, the orders, classes and phyla — 



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