148 



Species and Species Change 



Fig. 61, Inferred distribution of ancestral Madro-Tertiary plants in pre- 

 Eocene time, in which isolated populations became exposed to locally hotter 

 and drier climates. (After Axelrod.) 



the same geographic area. Competition for food could have re- 

 sulted in a restriction of the habitats of the species along local 

 ecological lines. Because the species would have had at least some 

 ecological differences, each would have tended to follow its own 

 advantages. With one species this could have led to greater suc- 

 cess in more open country, with another to greater success in more 

 heavily vegetated areas. In this fashion the ecological range of 



