viii author's preface 



and writings of others as possible without making the book 

 heavy ; but my aim has been to write a book to be read, not 

 merely one to be referred to. 



If it be asked, finally, for whom the book is intended, 

 I can hardly answer otherwise than ' For him whom it 

 interests.' The lectures were delivered to an audience con- 

 sisting for the most part of students of medicine and natural 

 science, but including some from other faculties, and some- 

 times even some of my colleagues in other departments. 

 In writing the book I have presupposed as little special 

 knowledge as possible, and I venture to hope that any one 

 who reads the book and does not merely skim it, will be 

 able without difficulty to enter into the abstruse questions 

 treated of in the later lectures. 



It would be a great satisfaction to me if this book were 

 to be the means of introducing my theoretical views more 

 freely among investigators, and to this end I have elaborated 

 special sections more fully than in the lectures. Notwith- 

 standing much controversy, I still regard its fundamental 

 features as correct, especially the assumption of ' controlling ' 

 vital units, the determinants, and their aggregation into 

 1 ids ' ; but the determinant theory also implies germinal 

 selection, and without it the whole idea of the guiding of 

 the course of transformation of the forms of life, through 

 selection which rejects the unfit and favours the more fit, 

 is, to my mind, a mere torso, or a tree without roots. 



I only know of two prominent workers of our day who 

 have given thorough-going adherence to my views : Emery 

 in Bologna and J. Arthur Thomson in Aberdeen. But 

 I still hope to be able to convince many others when the 

 consistency and the far-reachingness of these ideas are better 

 understood. In many details I may have made mistakes 

 which the investigations of the future will correct, but as 

 far as the basis of my theory is concerned I am confident : 

 the principle of selection does rule over all tlie categories of 

 vital units. It does not, indeed, create primary variations, 

 but it determines the paths of evolution which these are to 



