I PROGRESSION AND COMPETITION 19 



" . . . . Still better may we compare the vegetable kingdom 

 to a great tree branching from its base upwards, of which the 

 ends of the twigs represent the plant forms living at one time. 

 This tree has an enormous power of sprouting, and it would, if 

 it could develop without hindrance, form an inextricable bush- 

 like confusion of innumerable branchings. Extermination in 

 the struggle for existence, like a gardener, prunes the tree 

 continually, takes twigs and branches away, and produces 

 an orderly arrangement with clearly distinguishable parts. 

 Children who see the gardener daily at his task may well 

 suppose that he is the cause of the formation of the branches 

 and twigs. Yet the tree, without the constant pruning of 

 the gardener, would have been much greater, not in height, 

 but in extent, and in the richness and complexity of its 

 branching. 



"In the perfecting process (progression) and adaptation 

 lie the mechanical impulses which lead to the abundance of 

 forms; in competition and extermination, or in Darwinism 

 proper, only the mechanical cause of the formation of gaps in 

 the two organic kingdoms." 



Thus Nageli's theory attempts principally to explain two 

 points which that of Weismann leaves unexplained, namely, 

 the beginning of the formation of characters and the fact of 

 variation in definite directions. But Nageli's conception 

 seems to me to rest so much more on assumptions acutely 

 thought out than on facts, that it deserves rather to be 

 described as a materialistic-philosophical than as a mechanico- 

 physiological theory. 



