330 EXPERIMENTS WITH PLANTS 



leaves like ordinary plants and that their thickened, 

 leafless stems have been brought about by the dry 

 conditions under which they have lived, not directly 

 as in the Potato, but by a long -continued process of 

 change. We suppose that when the ancestors of the 

 Cactus first came to live under drier conditions than 

 they were accustomed to, some few of them were some- 

 what better adapted to stand drought and so .lived on 

 and propagated themselves under the new conditions, 

 while their fellows perished. Among the offspring of 

 these plants the same thing happened ; those best fitted 

 to the new conditions maintained themselves, while 

 the others perished. In each succeeding generation the 

 fittest 1 survived and crowded out the less fit in the 

 struggle for existence, the result being that in the 

 course of time the plant became better and better fitted 

 to its surroundings. In this way we suppose the vari- 

 ous forms of Cacti to have developed to their present 

 condition, this process of development being known 

 as evolution. It will be seen from the illustration that 

 evolution works by destroying the unfit 2 (thus giving 

 the fit a better chance to develop), and results in 

 bringing the plant into better and better harmony 

 with its surroundings, or environment. The process of 

 destroying the unfit and preserving the fit is called 



1 Whether the fittest originated gradually or suddenly (by mutation) is dis- 

 cussed in Chapter X. 



2 This, it should be said, is not the sole method by which evolution works, 

 but the only one of immediate importance in this connection. 



