INTRODUCTION. 7 



lution as a basis for contemporary enquiry into this subject, 

 but the fact remains that after Darwin, no one has set forth a 

 comprehensive theory of evolution worth the name. 



We will try to show, how on some questions which were al- 

 most wholly dark to Darwin, new light has been shed by later 

 facts. One of these questions, is that of the origin of variation. 

 It is clear that no evolution, no production of new species is 

 possible without variation of some kind. All the different theo- 

 ries of evolution start with variation. In the Lamarckian theo- 

 ry, variations are induced by the environment, and as the ef- 

 fect of this induction is thought to be directly transmittable, 

 species are gradually evolved, one from the other, by a continual 

 variation under the influence of the conditions under which the 

 species live. In the theories of de Vries, two kinds of variation 

 are distinguished, the small, individual variations of Lamarck, 

 induced by the environment, and not, or rarely transmittable, 

 and sudden variations of a more imposing kind, which have so 

 appreciable connection with environmental conditions, and 

 which are thought to be each the direct cause of the production 

 of a wholly new and complete species. 



In the Darwin — Weismann complex of theories, evolution is 

 thought to be caused by a continued natural selection on small 

 variations in all directions, the cause of which was a mystery 

 to Darwin, and is sought by Weismann in an indirect action of 

 the environment. Weismann's theory of evolution in its last 

 phases of development was essentially like that of Lamarck 



Some of the weakest points in all these theories of evolution 

 are these, that no sufficient account is made of variation, that 

 different kinds of variation are not distinguished, and that the 

 theories do not begin wflth the beginning, with the causes of 

 variation. 



Every theory of evolution must account for variation, it 

 must give a plausible explanation of the causes of that vari- 

 ation which may be instrumental in species-formation, and in the 

 second place it must account for specific stability. This second 

 point is also present in all the important theories of evolution. 



