Lines of Variation 
Darwin’s main ideas on variation separately, 
and to consider to what extent they seem to 
require modification in the light of later research. 
Firstly, Darwin believed that variations arise 
in what appears to be a haphazard manner, that 
they occur in all directions, and seem to be 
governed by the same laws as chance. It is 
our belief that we are now in a position to make 
more definite statements regarding variation than 
Darwin was able to. 
Biologists can now assert definitely that varia- 
tions do not always occur equally in all directions. 
The results of many years of the efforts of practi- 
cal breeders demonstrate this. These men have 
not been able to produce a green horse, a pigeon 
with alternate black and white feathers in the tail, 
or a cat with a trunk, for the simple reason that 
the organisms upon which they operated do not 
happen to have varied in the required direction. 
It may perhaps be objected that breeders have 
no desire to produce such forms; had they wished 
to do so, they would probably have succeeded. 
To this objection we may reply that they have 
not managed to produce many organisms, which 
would be highly desirable from a breeder’s point 
of view, as, for example, a blue rose, hens that lay 
brown eggs but do not become broody at certain 
seasons of the year, or a cat that cannot scratch. 
As Mivart well says, on page 118 of his Geneszs 
of Species, ‘‘ Not only does it appear that there are 
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