168 
DARWINISM 
CHAP. 
in those found in the same area hut frequenting somewhat 
different stations. 
To carry out these experiments with any satisfactory result, 
it will be necessary to avoid the evil effects of confinement 
and of too close interbreeding. If birds are experimented 
with, they should be allowed as much liberty as possible, a 
plot of ground with trees and bushes being enclosed with 
wire netting overhead so as to form a large open aviary. 
The species experimented with should be obtained in con¬ 
siderable numbers, and by two separate persons, each making 
the opposite reciprocal cross, as explained at p. 155. In the 
second generation these two stocks might lie themselves crossed 
to prevent the evil effects of too close interbreeding. By such 
experiments, carefully carried out with different groups of 
animals and plants, we should obtain a body of facts of a 
character now sadly wanting, and without which it is hopeless 
to expect to arrive at a complete solution of this difficidt 
problem. There are, however, some other aspects of the 
question that need to be considered, and some theoretical 
views which require to lie carefully examined, having done 
which avc shall be in a condition to state the general con¬ 
clusions to which the facts and reasonings at our command 
seem to point. 
Sterility due to changed Conditions and usually correlated with 
other Characters, especially with Colour. 
The evidence already adduced as to the extreme suscep¬ 
tibility of the reproductive system, and the curious irregu¬ 
larity with which infertility or sterility appears in the crosses 
between some varieties or species Avhile quite absent in those 
betAveen others, seem to indicate that sterility is a charac¬ 
teristic which has a constant tendency to appear, either by 
itself or in correlation Avith other characters. It is known 
to l»e especially liable to occur under changed conditions of 
life ; and, as such change is usually the starting-point and 
cause of the development of ucav species, avc have already 
found a reason why it should so often appear Avhen species 
become fully differentiated. 
In almost all the cases of infertility or sterility betAveen 
varieties or species, avc have some external differences Avith 
