210 GENERAL BOTANY 



best adapted to the conditions survive, so that the cacti, the bul- 

 rushes, and the algse are in a true sense the fittest survivors, 

 selected by the extreme conditions of their environment. 



These habitats are not unlike a very exacting plant breeder, 

 who ruthlessly discards all plants in his culture plots which do 

 not conform to a rigid ideal which he has started out to attain. 

 In the development of cultivated races from fluctuating varia- 

 tions it has been found, however, as has already been stated, that 

 such races are both unstable and limited in the degree to which 

 a giv,en variation or tendency can be developed. If the experi- 

 ence of the future confirms that of the past, it would seem to 

 furnish a formidable objection to Darwin's conception that in 

 wild nature fluctuating variations may be accumulated until a 

 new species is formed. 



Selection of mutations. Darwin was aware of the fact that 

 sports, or mutations, occasionally arose in nature, but he did not 

 think that they occurred with sufficient frequency or in sufficient 

 numbers to be of great importance in the evolution of wild 

 species. The experiments and writings of De Vries, however, 

 and the experience of breeders and botanists who have been stim- 

 ulated by his discoveries, seem to indicate that mutations may be 

 a more important factor in evolution than Darwin supposed. 

 Variation and selection would here, as in evolution through 

 fluctuating variations, be the fundamental principles concerned 

 in the origin and perpetuation of improved varieties or species. 

 " Species," says De Vries, " are derived from other species by 

 means of sudden small changes, which, in some instances, may 

 be scarcely perceptible to the inexperienced eye. From their 

 first appearance they are uniform and constant when propagated 

 by seed ; they are not connected with the parent species by inter- 

 mediates and have no period of slow development before they 

 reach the full display of their characters." Some young species 

 will be better fitted for their life conditions than others, and the 

 struggle for life will induce a selection among them by which 

 the fittest survive. And again : " Thus we come to the conclu- 

 sion that natural selection is as active as Darwin assumed it to 

 be, and is as preeminent a factor in evolution. It causes the 



