VI PREFACE 



are urged with much force and more confidence, and for the 

 most part by the modern school of laboratory naturalists, to 

 whom the peculiarities and distinctions of species, as such, 

 their distribution and their affinities, have little interest as 

 compared with the problems of histology and embryology, 

 of physiology and morphology. Their work in these depart- 

 ments is of the greatest interest and of the highest importance, 

 but it is not the kind of work which, by itself, enables one to 

 form a sound judgment on the questions involved in the 

 action of the law of natural selection. These rest mainly on 

 the external and vital relations of species to species in a state 

 of nature — on what has been well termed by Semper the 

 " physiology of organisms," rather than on the anatomy or 

 physiology of organs. 



It has always been considered a weakness in Darwin's 

 work that he based his theory, primarily, on the evidence of 

 variation in domesticated animals and cultivated plants. I 

 have endeavoured to secure a firm foundation for the theory 

 in the variations of organisms in a state of nature ; and as 

 the exact amount and precise character of these variations is 

 of paramount importance in the numerous problems that 

 arise when we apply the theory to explain the facts of nature, 

 I have endeavoured, by means of a series of diagrams, to 

 exhibit to the eye the actual variations as they are found to 

 exist in a sufficient number of species. By doing this, not 

 only does the reader obtain a better and more precise idea of 

 variation than can be given by any number of tabular state- 

 ments or cases of extreme individual variation, but we obtain 

 a basis of fact by which to test the statements and objections 

 usually put forth on the subject of specific variability ; and it 

 will be found that, throughout the work, I have frequently to 

 appeal to these diagrams and the facts they illustrate, just as 

 Darwin was accustomed to appeal to the facts of variation 

 among dogs and pigeons. 



