THE EVOLUTION OF LIFE 



demonstrate the truth of his theory of mutation. The 

 warm approval which it won from a number of eminent 

 botanists, and especially vegetal physiologists, was not 

 shared by zoologists. Of these Weismann, in his Lect- 

 ures on the Theory of Descent (1902, ii. p. 358), and Plate 

 in his Problems of Species-formation (1903, p. 174), have 

 dealt fully with the theory of mutation, and, while ap- 

 preciating the interesting observations and experiments 

 of De Bries, have rejected the theory he has built on 

 them. As I share their opinion, I may refer the reader 

 who is interested in these difficult problems to their 

 works, and will restrict myself here to the following 

 observations. The chief weakness of the theory of 

 mutation of De Bries is on its logical side, in his dog- 

 matic distinction between species and variety, mutation 

 and variation. When he holds the constancy of species 

 as a fundamental "fact of observation," we can only 

 say that this (relative) permanence of species is very 

 different in the different classes. In many classes (for 

 instance, insects, birds, many orchids and graminea) 

 we may examine thousands of specimens of a species 

 without finding any individual differences; in other 

 classes (such as sponges, corals, in the genera rubus and 

 hieracium) the variability is so great that classifiers 

 hesitate to draw up fixed species. The marked differ- 

 ence between various forms of variability which De Bries 

 alleges cannot be carried through ; the fluctuating varia- 

 tions (which he takes to be unimportant) cannot be 

 sharply distinguished from the abrupt mutations (from 

 which new species are supposed to result at a bound). 

 De Bries's mutations (which I distinguished in the Gen- 

 eral Morphology as "monstrous changes" from other 

 kinds of variation) must not be confused with the pale- 

 ontological mutations of Waagen (1869) and Scott 

 (1894) which have the same name. The sudden and 

 striking changes of habit which De Bries observed only 



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