INTRODUCTION. 3 



interested themselves, like Darwin, in genetical questions for the 

 sake of their general aspect. The first author after Darwin who 

 approached Genetics in this spirit was Bateson. Nearly all the 

 Biologists who have interested themselves in Genetics have 

 been Zoologists or Botanists first, and Gene icians in the second 

 place. Because, until recently, there existed in Genetics no 

 points of contact with other branches of Biological science, no 

 guiding principles, the work of the older Geneticians stands iso- 

 lated in a remarkable way. Each of them tried to construct for 

 himself a complete conception of the mechanism of variation 

 of the inheritance of different qualities from parents to child- 

 ren, and of the causes for the diversity of animals and plants 

 which inhabit the earth. 



Most of these older Geneticians have been either Zoologists 

 orBotanists, and we will see how the very fact that one is Zo5- 

 logist or Botanist exclusively can hinder one in the making of 

 his generalizations. This drawback comes to light directly we 

 compare Darwin's generalizations with those of his followers, 

 who, with the exception of Bateson, did not, as Darwin did, 

 have an open eye for the most diverse facts of variation, and 

 the influence of selection in animals as well as in plants. For 

 instance, if we compare the ideas about evolution of the Zoolo- 

 gist Weismann and the Botanist de Vries. 



There is one point in common to all the theories of evolution, 

 excepting Darwin's, and that is, that each theoretician has 

 always over-emphasized one point, one single link in the chain 

 of processes which goes to the making of species, and has 

 brought out this point as "the" cause of evolution. Just as 

 Lamarck has given all his attention to adaptation, and has led 

 himself to believe adaptation to be the cause of numerous pro- 

 cesses with which we now believe it to be only remotely con- 

 cerned, so has Weismann over emphasized the power of select- 

 ion to the exclusion of everything else. And de Vries, who 

 believed he had witnessed a striking instance of mutation, 

 spontaneous origin of species, has come to believe mutation to 

 be the sole important cause of evolution. 



