PREFACE vii 



and Hfematococcus in considerable detail and used these familiar 

 organisms as pegs on which to hang some elementary ideas with 

 regard to the nature of living things and the differences between 

 animals and plants. Otherwise I have as far as possible avoided 

 the type-system as being altogether unsuitable for a work of 

 this kind, though of course I have been obliged to refer to 

 numerous different organisms in illustration of special points. 



It was only by rigidly excluding from the earlier part of the 

 book everything that was not considered essential to the under- 

 standing of general principles that it has been possible to find 

 space for even a brief presentation of the evidence upon which 

 the theory of organic evolution rests, and for a discussion of the 

 principal factors which appear to have co-operated in determining 

 the course of that evolution. 



Although the entire work is intended to be of an elementary 

 character, it has been impossible, in connection with the theory 

 of heredity, to avoid, on the one hand, a considerable amount of 

 cytological detail, and, on the other, some discussion of theoretical 

 speculations of a highly controversial nature. In dealing with 

 these vexed questions, which underlie the whole problem of 

 organic evolution, I have endeavoured to present the views of 

 opposing schools of thought as fairly as possible, but I must 

 confess that I have ventured to lay considerable stress upon 

 ideas which, though widely accepted elsewhere, have not as yet 

 met with much appreciation in this country, though that they 

 will do so in the future can hardly be doubted. 



By way of introduction to the discussion of the factors of 

 organic evolution a chapter has been devoted to the views of 

 Buffon, Erasmus Darwin and Lamarck, and another to those of 

 Charles Darwin, Robert Chambers and Alfred Russel Wallace, 

 and I have endeavoured to present the opinions of these classical 

 authors as far as possible by means of quotations from their 

 own writings. 



In a work such as the present the employment of numerous 

 technical terms is, of course, unavoidable, but it is hoped that 

 the meaning of these is sufficiently explained in the text, and the 

 use of the index should obviate any difficulties in this respect, 

 especially if the book is read systematically from the first chapter 

 onwards. 



It is a pleasure to record my thanks to many colleagues who 

 have ungrudgingly helped me in various ways. Amongst these 



