180 THE PROBLEMS OF EVOLUTION 



similar opportunism is manifest. The ancient 

 Crossopterygii could live out of the water, and 

 outside of the shrinking streams they found less 

 severe competition with other creatures. More- 

 over the shrinkage of the hydrosphere left greater 

 opportunity for life in the transitional zone and 

 so placed a premium on their utilization of their 

 capacity for semiterrestrial life. The arboreal 

 ancestors of man could live on the ground and so 

 were not forced to retreat with the tropical forests 

 in order to maintain themselves. In every known 

 case of evolution, the capacity for life under new 

 conditions has been present and the new conditions 

 have become available. The one factor is as neces- 

 sary as the other, and their coincidence establishes 

 an opportunity which has, apparently, seldom been 

 refused. 



Non-adaptive characters are no less a result of 

 opportunity in the sense that inherent possibilities 

 and favorable circumstances are necessary to the 

 production of any organic entity. The colors of 

 butterflies' wings are due to hereditary factors, 

 including the ability of the body to form the nec- 

 essary pigments in the case of pigmental colors, but 

 in the wet- and dry-season forms previously men- 

 tioned we see that the external environment may 

 also play a direct part in the production of a 

 given color or pattern. The factors of heritage and 

 internal environment have certain potentialities; 



