386 THE BIOLOGY OF BIRDS 



the numbers of each class of young produced. Then the 

 cocks were subjected to alcohol inhalation, and thereafter 

 mated again. The record of the different number of each 

 class of young produced was different from before, indicating 

 that the administration of the alcohol altered the propor- 

 tions, and this could be done only by a discriminate 

 selection of certain classes of spermatozoa. 



§ 9. Isolation and the Reverse : In-breeding and 

 Out-breeding 



There is another directive factor in evolution, which 

 operates on variations that crop up. This factor is called 

 " isolation," and it includes all the means that restrict the 

 range of inter-crossing within a species. The barriers may 

 be spatial, as when a peninsula becomes an island, or 

 temporal, when different members of the species become 

 mature at different times. They may be habitudinal, when 

 different members of the species adopt different ways of 

 living. They may be physiological, when differences in size, 

 for instance, prevent crossing between dwarfs and giants. 

 They may be psychical, e.g. when different varieties, say of 

 . pigeon, show marked preferences in their mating, seeking 

 out sometimes those of their own colour. As examples of 

 isolation among birds we may cite the case of the Golden 

 Warblers of the West Indies, for each well-separated island 

 seems to have its own form or species. Similarly, each 

 island in Hawaii has its own species of a number of genera of 

 Drepanine birds. 



Isolation as a factor in evolution must operate in two ways, 

 for we cannot separate a shortening of the radius of possible 

 crossing from a lengthening of it. When a peninsula 

 becomes an island by a coastal depression there is a reduction 

 of the possible inter-crossings among the members of a 

 species. When a coastal elevation joins an island to the 

 mainland there is an increase of the possible inter-crossings. 



On the one hand, there is increased in-breeding or 

 endogamy, which tends to make a stock more homogeneous 



