PLANT AFFINITIES 215 



evolved in past ages by a development from 

 earlier forms, but you very probably supposed 

 that this creative process has now come to a 

 standstill. Let me assure you, then, that this 

 process is going on to-day very actively, in all 

 probability quite as actively as at any time in 

 the past. 



"Species of plants in a state of nature are 

 constantly crossing and new species are being 

 developed under our eyes. 



"There is nothing anomalous about the case 

 of the tarweeds, although they afford a very 

 interesting illustration of the development of 

 which I speak. The same thing may be ob- 

 served in the case of certain genera of the mint 

 family. Here in some cases the hybrids thrive 

 almost to the entire exclusion of the parent 

 species. In other cases they gradually dis- 

 appear, being too unstable to establish them- 

 selves by seed. 



"Everything, of course, depends upon the 

 qualities of the hybrid. If it is well adapted to 

 the environment it survives. If better adapted 

 than its parents, it probably displaces them alto- 

 gether. But, on the other hand, if the hybrid is 

 less well adapted than the parent forms to make 

 its way in the world, it is of course weeded out 

 by natural selection." 



