6 INTRODUCTION 
by the action of natural selection upon minute differ- 
ences, in the manner which Darwin described, and 
which has been claimed by others as the sole means 
by which the origin of new forms takes place. The 
evidences of evolution are much more numerous and 
more weighty than the evidences of the survival of 
the fittest. The theory of evolution, as opposed to 
the creation hypothesis, is supported by innumerable 
facts of classification, of morphology, and of embryo- 
logy, by the geographical distribution of animals and 
plants, and by their succession in the geological strata, 
as well as by direct observation of the actual occurrence 
of changes in the case of domestic productions as well 
as under Nature, and many of these facts have no 
direct bearing upon the theory of natural selection. 
Before discussing the problem of the origin of species, 
it is necessary to arrive at some idea as to what the 
term ‘species’ means. And this is not altogether an 
easy matter, since a precise definition has not been, 
and cannot be, agreed upon. The idea of species is, 
indeed, of great antiquity and very gradual growth. 
Primitive men doubtless recognised certain plants or 
animals as being like one another, and different from 
others, and they gradually came to distinguish such 
forms by giving a different name to each. The names 
first used must have applied as a rule to genera rather 
than to species. Thus, such common names of plants 
as rose, bramble, vetch, nettle, dock, crowfoot, are 
names of genera—groups of greater extent than species, 
and often more easily defined than the latter. Later 
