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 A r ariation 
among dogs and pigeons. 
