CHAPTER XXIX. 



CHOROLOGY, OR THE PRINCIPLES OF THE GEOGRAPHICAL DIS- 

 TRIBUTION OF ANIMALS AND PLANTS. 



Having briefly considered the life districts of the habitable 

 earth, and the bionomic characters of plants and animals, we may 

 now enquire into the laws which govern the geographical distribu- 

 tion of organisms. It is clear that, whatever the present distribution 

 of plant and animal life, it has not always been so. Even the most 

 cosmopolitan species had its circumscribed center of origin, for 

 it is extremely unlikely that the same species originated in more 

 than one locality.* From this locality, its center of dispersion, it 

 spread to occupy whatever territory was available. Occupancy of 

 new territory, however, is possible only if this territory corresponds 

 in facies to that from which the species is derived, in the degree 

 in which the species is dependent upon the facies. Hence the exo- 

 dists from their land of birth are not always able to find a suitable 

 place of settlement, and, though their numbers may be vast, a 

 large proportion is sure to perish. 



The factor of greatest importance in determining whether or 

 not an area is to be permanently occupied by the members of a 

 newly arrived species is the organic factor, or the question of food 

 supply and protection from enemies. If the food supply is insuf- 

 ficient, or if contending species hold the ground, the new arrivals 

 may be prevented from occupying the territory, even though the 

 facies is otherwise adapted to their needs. 



* Strictly speaking, of course, a species is monophyletic and can arise only 

 in one locality. But types so similar that they may easily be mistaken for the 

 same species may arise in different localities. Thus, what appears to be the same 

 species of Fusus (F. closter) occurs in the West Indies and in the Red Sea, but 

 they are most probably of independent origin. Organisms of such similar 

 characteristics that they are commonly mistaken for members of the same genus 

 are frequently met with in widely separated localities, where they have arisen 

 independently. The independent origin of horses in America and in the Old 

 World was advocated by Cope but rejected by others. Some widely separated 

 pulmonate gastropods, placed in the same genus, may have arisen independently 

 from marine ancestors. 



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