444 DARWINISM chap, xiv 



The same remark applies to the views of Professor Geddes on 

 the laws of growth which have determined certain essential 

 features in the morphology of plants and animals. The 

 attempt to substitute these laws for those of variation and 

 natural selection has failed in cases where we can apply a 

 definite test, as in that of the origin of spines on trees and 

 shrubs ; while the extreme diversity of vegetable structure 

 and form among the plants of the same country and of the 

 same natural order, of itself affords a proof of the j^reponder- 

 ating influence of variation and natural selection in keeping 

 the many diverse forms in harmony with the highly complex 

 and ever-changing environment. 



Lastly, we have seen that Professor Weismann's theory of 

 the continuity of the germ -plasm and the consequent non- 

 heredity of acquired characters, while in perfect harmony 

 with all the well-ascertained facts of heredity and development, 

 adds greatly to the importance of natural selection as the one 

 invariable and ever-present factor in all organic change, and 

 that which can alone have produced the temi)orary fixity 

 combined with the secular modification of species. While 

 admitting, as Darwin always admitted, the co-operation of the 

 fundamental laws of growth and variation, of correlation and 

 heredity, in determining the direction of lines of variation 

 or in the initiation of peculiar organs, we find that variation 

 and natural selection are ever-23resent agencies, which take 

 possession, as it were, of every minute change originated 

 by these fundamental causes, check or favour their further 

 development, or modify them in countless varied ways 

 according to the varying needs of the organism. AVhatever 

 other causes have been at work. Natural Selection is su})reme, 

 to an extent which even Darwin himself hesitated to claim 

 fur it. The more we study it the more we are convinced of 

 its overpowering importance, and the more confidently we 

 claim, in Darwin's own words, that it " has been the most 

 important, but not the exclusive, means of modification." 



