THE COURSE < »!•' BIOLOGIC EVOLUTION. 39 



stricted in range, reduced in number, and nearly crowded out 

 of existence. 



We have seen that the deterioration or extinction can be 

 only brought about by a change of environment. The only 

 cause for the predominance of a type is its greater adaptation 

 to the existing environment. If undisturbed any given type 

 of structure will equilibrate in the direction of greater adapta- 

 tion until this is no longer possible. But complete adapta- 

 tion, as I long ago pointed out,* is impossible. It is always 

 possible for a new type to appear which shall respond more 

 exactly to the surrounding conditions. The environment, it is 

 true, may undergo unfavorable changes. The climate may 

 change, or the type in its migrations may encounter unfriendly 

 influences. Most effective of all is the ever-changing influence 

 of the contemporary life with which a type must come into 

 competition. It must, as we have seen, eventually encounter as 

 a rival in the race for life, the new type which is to succeed it. 

 endowed with elements of new life and with fresh powers both 

 to overcome hostile influences and to utilize the resources of na- 

 ture. Such superior types, as already shown, are ever and 

 anon arising, proceeding frcm quarters least anticipated, ap- 

 pearing without regularity either as to place or time, springing 

 sympodially frcm the original trunk, rising impiously above 

 their parents, and ultimately overshadowing, repressing, crush- 

 ing, and extinguishing the former lords of the vegetable king- 

 dom. Such in brief is the generalized history of the rise and 

 fall of empires in the world of plants. 



What has thus far been said is perhaps .sufficient to render 

 clear to most minds the peculiar and complicated character of 

 biologic evolution in general, and to show how widely it differs 



• ;; American Naturalist. February 1881, p. 89. 



