Chap. IV. 
TO NATURAL SELECTION. 
103 
this case the effects of intercrossing can hardly be coun¬ 
terbalanced by natural selection always tending to mo¬ 
dify all the individuals in each district in exactly the 
same manner to the conditions of each; for in a con¬ 
tinuous area, the physical conditions at least will gene¬ 
rally graduate away insensibly from one district to 
another. The intercrossing will most affect those ani¬ 
mals which unite for each birth, which wander much, 
and which do not breed at a very quick rate. Hence 
in animals of this nature, for instance in birds, varieties 
will generally be confined to separated countries; and 
this I believe to be the case. In hermaphrodite organ¬ 
isms which cross only occasionally, and likewise in 
animals which unite for each birth, but which wander 
little and which can increase at a very rapid rate, a 
new and improved variety might be quickly formed on 
any one spot, and might there maintain itself, in a 
body, so that whatever intercrossing took place would 
be chiefiy between the individuals of the same new 
variety. A local variety when once thus formed might 
subsequently slowly spread to other districts. On the 
above principle, nurserymen always prefer getting seed 
from a large body of plants of the same variety, as 
the chance of intercrossing with other varieties is thus 
lessened. 
Even in the case of slow-breeding animals, which 
unite for each birth, we must not overrate the effects 
of intercrosses in retarding natural selection ; for I can 
bring a considerable catalogue of facts, showing that 
within the same area, varieties of the same animal can 
long remain distinct, from haunting different stations, 
from breeding at slightly different seasons, or from 
varieties of the same kind preferring to pair together. 
Intercrossing plays a very important part in nature 
in keeping the individuals of the same species, or of the 
same variety, true and uniform in character. It will 
