and their affinities, have little interest as compared with the problems
of histology and embryology, of physiology and morphology. Their work in
these departments 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 statements 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.