CHAPTER XIV 



THE THEORY OF EVOLUTION 



In the preceding chapters we have examined the evidence 

 concerning the influence of external conditions in bringing about 

 changes in the structure of animals; we have examined the 

 claim that acquired characters are inherited ; we have also 

 studied the results from hybridizing in Mendelian and other 

 cases, and we have considered the problems connected with the 

 differences between the inheritance of fluctuating (or continu- 

 ous) and of definite (or discontinuous) variations. The evi- 

 dence from these different sources has unquestionably a very 

 direct bearing on the problem of organic evolution, yet so many 

 questions are still unsettled that any conclusion drawn from the 

 evidence that we now possess must be provisional and perhaps 

 premature. It is with this understanding that I venture on the 

 following analysis of the bearing of the evidence on the theory of 

 evolution. That the process of evolution is complex, and that 

 many conditions and several kinds of variation may have con- 

 tributed to it, most zoologists seem at the present time willing to 

 admit. If in the following pages I have laid especially emphasis 

 on the results of recent experimental work on discontinuous 

 variation and inheritance, it is because we have here, I think, the 

 most satisfactory evidence in regard to certain factors that may 

 have played an important role in the process of evolution. Other 

 factors may also have been involved, but we have insufficient 

 experimental evidence to prove that this is the case. 



The formulation of the modern theory of evolution we owe 

 chiefly to the French naturaUsts, Buffon, Lamarck, and Geoffroy 

 St. Hilaire; to some extent also to Erasmus Darwin. Their 

 theories were based largely on evidence from comparative 



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