450 PLANT DISTRIBUTION 



Moreover, he is aware that all organisms have various 

 means of adjusting themselves to their environment. 

 These three factors account in a general way for the 

 fundamental facts of distribution, namely: (1) that 

 there are different assemblages of organisms in distant 

 regions, (2) that species are more or less widely distri- 

 buted over the world, and (3) that plants and animals 

 are aggregated into groups adapted to live under similar 

 environmental conditions. Each of these factors may 

 now receive a somewhat fuller consideration in order 

 to see just how they actually work out. 



Evolution and Distribution. — When new species 

 originate by evolution it is probable that in the vast 

 majority of cases they do so in one particular locality 

 only. From this they spread more or less widely accord- 

 ing to circumstances. In the great majority of cases they 

 find something to prevent their spread over the whole 

 world, from which it follows naturally that evolution 

 tends to create diversity among the plants and animals 

 of widely separated regions. It can be readily seen that 

 this would be true even if all parts of the earth had 

 been originally populated with exactly the same species; 

 evolution of new and different species in regions distant 

 from one another would in the course of time have given 

 them widely different assemblages of plants and animals. 



Migration and Distribution. — When new species 

 of plants and animals have once evolved they spread 

 gradually away from their place of origin. Some sorts of 

 animals and plants are able to spread rapidly, as, for 

 example, birds and dandelions. On the other hand angle- 

 worms and plants with large heavy seeds have in nature 

 very much less efficient means of dispersal. From illus- 

 trations of this sort it is easy to understand that the 

 means of dispersal govern in some measure the probabil- 

 ity of a particular organism spreading to the limits of its 

 possible range. However, it must be noted that even 

 the organisms with the least eflficient means of dispersal 



