vi PREFACE 



have taken place in biological thought and procedure since 

 his time. We do not suggest that the attempts at a synthetic 

 treatment that have been made in recent years are to be lightly 

 disregarded. To some of these, indeed, we are deeply indebted. 

 We feel, however, that many new and fundamental questions 

 are left still unrelated one with another. Moreover, no 

 adequate attempt has been made to see how far the data 

 of variation in structure . and behaviour confirm particular 

 theories of evolution. 1 



We are under obligation to numerous fellow workers who 

 have given time and trouble in assisting us, and tender our 

 thanks for their generous help and the trouble they have taken 

 on our behalf. 



G. C. R. O. W. R. 



1 Owing to the illness of one of the authors the publication of this book was 

 delayed, and no references to literature later than 1 933 are included. 



