168 DARWINISM 



in those found in the same area but 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 be 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 difficult 

 problem. There are, however, some other aspects of the 

 question that need to be considered, and some theoretical 

 views which require to be carefully examined, having done 

 which we 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 while quite absent in those 

 between others, seem to indicate that sterility is a charac- 

 teristic which has a constant tendency to appear, either by 

 itself or in correlation with other characters. It is known 

 to be especially liable to occur under changed conditions of 

 life ; and, as such change is usually the starting-point and 

 cause of the development of new species, we have already 

 found a reason why it should so often appear when species 

 become fully differentiated. 



In almost all the cases of infertility or sterility between 

 varieties or species, we have some external differences with 



