IV 
VARIATION UNDER DOMESTICATION 
99 
Although I believe, for reasons which will be given further 
on, that some amount of variability is a constant and 
necessary property of all organisms, yet there appears to be 
good evidence to show that changed conditions of life tend to 
increase it, both by a direct action on the organisation and 
by indirectly affecting the reproductive system. Hence the 
extension of civilisation, by favouring domestication under 
altered conditions, facilitates the process of modification. Yet 
this change does not seem to be an essential condition, for 
nowhere has the production of extreme varieties of plants and 
flowers been carried farther than in Japan, where careful 
selection continued for many generations must have been the 
chief factor. The effect of occasional crosses often results in 
a great amount of variation, but it also leads to instability of 
character, and is therefore very little employed in the pro¬ 
duction of fixed and well-marked races. For this purpose, in 
fact, it has to be carefully avoided, as it is only by isolation and 
pure breeding that any specially desired qualities can be in¬ 
creased by selection. It is for this reason that among savage 
peoples, whose animals run half wild, little improvement takes 
place ; and the difficulty of isolation also explains why distinct 
and pure breeds of cats are so rarely met with. The wide dis¬ 
tribution of useful animals and plants from a very remote 
epoch has, no doubt, been a powerful cause of modification, 
because the particular breed first introduced into each country 
has often been kept pure for many years, and has also been 
subjected to slight differences of conditions. It will also 
usually have been selected for a somewhat different purpose 
in each locality, and thus very distinct races would soon 
originate. 
The important physiological effects of crossing breeds or 
strains, and the part this plays in the economy of nature, will 
be explained in a future chapter. 
Concluding Remarks. 
The examples of variation now adduced — and these might 
have been almost indefinitely increased— will suffice to show 
that there is hardly an organ or a quality in plants or animals 
which has not been observed to vary; and further, that when¬ 
ever any of these variations have been useful to man he has 
