PRESENT ASPECTS OF EVOLUTION 280 



future generations as have resulted from modifications 

 arising in the idioplasm. Descendants of plants which 

 have varied under changed conditions revert, as a rule, 

 to the old type, when returned to the old surroundings. 

 And in the animal world effects of use and disuse are, 

 according to his view, not transmissible. 



Natural selection plays a very subordinate part. It 

 is purely destructive. Given an infinity of place and 

 nourishment do away, that is, with all struggle and 

 selection and the living world would have advanced, 

 purely by the force of the progressive tendency, just 

 as far as it now has ; only there would have survived 

 an indefinite number of intermediate forms. It would 

 have differed from our present living world as the 

 milky way does from the starry firmament. 



He compares the plant kingdom to a great, luxu- 

 rious tree, branching from its very base, whose twigs 

 would represent the present stage of our different 

 species. Left to itself it would put out a chaos of in- 

 numerable branches. Natural selection, like a gar- 

 dener, prunes the tree into shape. Childen might im- 

 agine that the gardener caused the growth ; but the 

 tree would have been broader and have branched more 

 luxuriantly if left to itself.* 



Every species must vary perpetually. Now this 

 proposition is apparently not in accord with fact ; for 

 some have remained unchanged during immense pe- 

 riods. And natural selection, by removing the less fit, 

 certainly appears to contribute to progress by raising 

 the average of the species. The theory seems extreme 

 and one-sided. And yet it has done great service by 



* See Nageli, " Theorie der Abstammungslehre," p. 18 ; also pp. 12, 118, 

 285. 



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