128 THE SOCIAL LIFE OF ANIMALS 



population was homogeneous, and when mutation 

 rates are similar throughout, even though selection is 

 in the same direction in all parts of the different 

 colonies. The primary factor under these conditions 

 will be that of effective breeding population size, and 

 there will be greater chance for varied evolution 

 among the populations that are intermediate in size, 

 as contrasted with those which are small or large, and 

 still greater chance for evolution when a large species 

 is broken into small breeding colonies which are not 

 completely isolated from each other. 



This argument, even as I have simplified it, is not 

 too easily followed the first time one goes over it. 

 Perhaps my use of an old teaching trick, that of repe- 

 tition of the same ideas with different words and 

 different illustrations, may be forgiven. In doing so 

 I am still leaning heavily on Professor Wright. The 

 series of diagrams shown in Plate IV are built on 

 one fundamental background. In perspective we see 

 two elevations, one higher than the other, and two 

 depressions which are the low points in a valley 

 between the two peaks. Every position is intended 

 to represent a different combination of gene fre- 

 quencies. The peaks represent gene combinations 

 which are highly adaptive; the depressions represent 

 those that lack adaptive value. The degree of adap- 

 tiveness is shown by the height occupied by the given 



