2 INTRODUCTION 



differences are to be found between different members 

 of the same family. Some of these differences arise 

 comparatively late in life, and may be the result of 

 circumstances or of education. It is the first duty of 

 the student of variation to distinguish as far as may 

 be possible between differences of this kind on the one 

 hand, and those differences on the other which depend 

 upon the fact that the different detached fragments, 

 as we have termed them, of the parent organism its 

 germ-cells, in fact show greater or smaller differences 

 among themselves. 



The facts of variation have this very special impor- 

 tance, that the whole theory of organic evolution is 

 based upon them. The fact that members of the same 

 species are not all alike, depending upon the further 

 fact that offspring may differ from their parents, makes 

 it possible in the course of generations for progressive 

 changes to take place, so that from the offspring of 

 different members of the same species different new 

 species may arise. But for this fact of variation it 

 would have been quite impossible for Darwin to have 

 overthrown the former crude belief in a special crea- 

 tion of each separate species, since there would have 

 been no material for his great factor natural selection 

 to work upon. It is with variation, then, and with 

 the manner in which characters appear in the succes- 

 sive generations of living things, that we are here 

 concerned. 



Ever since the publication of Darwin's ' Origin of 

 Species ' in 1859, these subjects, and especially the 

 theoretical aspects of them, have been received even 



