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 zodlogists seem at the present time willing to 
admit. Ifin 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 réle 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 naturalists, 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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