iv INFLUENCE OF ENVIRONMENT 179 



as well as in animals. It is because plants and 

 animals vary naturally or spontaneously here spon- 

 taneously merely means from unknown causes that 

 man has been able to select among the variations 

 and to make them become permanent. And when 

 we see how very different are the lines along which 

 the same species has varied take the cabbage, for 

 instance, or the dog we are warranted in drawing the 

 conclusion that variability is very considerable among 

 cultivated or domesticated organisms. 1 



We must now consider the last of the three groups 

 of facts which lie at the basis of experimental trans- 

 formism. We have considered variability in the state 

 of nature, and shown that it is to be met with in all 

 parts of the organism ; we have also shown that the 

 same variability obtains among cultivated plants and 

 domestic animals whose numerous varieties are the 

 result of man's selection. We must now refer to the 

 facts which show how natural variability may be 

 determined or facilitated. These facts may be 

 arranged under the general heading of the influence 

 of environment in the production of variations. 2 Their 



1 See De Candolle's Origine des Plantes Cultivees, and Sturtevant's 

 important series of articles on Originof Cultivated Plants, in American 

 Naturalist y 1887-89. 



- Besides Samper's well-known Animal Life (published in the Inter- 

 national Science Series], the reader will find a large body of subsequent 

 literature condensed in J. Arthur Thomson's Synthetic Summary of the 



N 2 



