2 INTRODUCTION 
differences are to be found between different members 
of the same family. Some of these differences arise 
comparatively late in life, and may be the result of 
circumstances or of education. It is the first duty of 
the student of variation to distinguish as far as may 
be possible between differences of this kind on the one 
hand, and those differences on the other which depend 
upon the fact that the different detached fragments, 
as we have termed them, of the parent organism—its 
germ-cells, in fact—show greater or smaller differences 
among themselves. 
The facts of variation have this very special impor- 
tance, that the whole theory of organic evolution is 
based upon them. The fact that members of the same 
species are not all alike, depending upon the further 
fact that offspring may differ from their parents, makes 
it possible in the course of generations for progressive 
changes to take place, so that from the offspring of 
different members of the same species different new 
species may arise. But for this fact of variation it 
would have been quite impossible for Darwin to have 
overthrown the former crude belief in a special crea- 
tion of each separate species, since there would have 
been no material for his great factor—natural selection 
—to work upon. It is with variation, then, and with 
the manner in which characters appear in the succes- 
sive generations of living things, that we are here 
concerned. 
Ever since the publication of Darwin’s ‘Origin of 
Species’ in 1859, these subjects, and especially the 
theoretical aspects of them, have been received even 
