212 Robertson : Theortes of Evolution. 
generation which possessed in the highest degree some special 
quality at which the breeder was aiming. The material upon 
which the breeder has to work is the variations which 
spontaneously arise. He cannot compel a variation in the 
direction which he desires, but he can make use of it if it 
happens to arise. The variability of domesticated plants and 
animals is particularly marked, but species in the wild condition 
vary much more than is generally supposed. Misapprehension 
on this point arises from the fact that systematists are inclined 
to take no account of variations from the type form of a species 
unless the variation is sufficiently well marked to be named and 
classified as a sub-species or variety. Darwin held that between 
species and variety there is no hard and fast line. If this is 
true, and species are only strongly marked and well defined 
varieties, it follows that species belonging to a large genus 
will be more likely to present varieties than species belonging 
toa small genus. For a large genus is a group within which 
the manufactory of species in past times went on with special 
vigour, the numerous species of the genus existing at the 
present day being a legacy from this former activity. If this 
activity is not only a matter of the past, but continues into 
modern times, we shall find a number of species now actually 
in the making—that is, many of the species in the genus will 
show sub-species and varieties. Darwin tested this sup- 
position by tabulating the plants and beetles of twelve 
countries according to the size of the genera and the number of 
recorded varieties, and found that it was borne out by the facts. 
If we grant that variations exist, and that no sharp line 
can be drawn between species and varieties, the question next 
arises—how is it that varieties become so far differentiated from 
their parent species as to be converted into fixed and stable 
species, and how is it that these species show such wonderful 
adaptations to their surroundings? In other words—what 
factor in the natural evolution of species takes the place of that 
‘artifical selection’ by which man produces his domestic races ? 
The answer is that this factor is the ‘ natural selection’ brought 
about by the struggle for existence between living things. 
Since many more animals and plants are brought into the world 
than can possibly survive, variations, however slight, which 
happen to be in any degree profitable to the individuals 
exhibiting them, will tend to the preservation of such individuals, 
and will very likely be inherited by their offspring. This pro- 
cess is conveniently known by Herbert Spencer’s name of the 
Naturalist, 
