AGGREGATIONS OF HIGHER ANIMALS 129 



population. The variability of the population is 

 shown by the size of the area that is occupied. Every 

 individual in a species may have a different gene 

 combination from every other, and yet the species 

 may occupy a small region relative to all the possi- 

 bilities. 



We may call the lower peak Mount Minor Adap- 

 tation and the higher one Mount Major Adaptation. 

 In Figure A we find a population which is fairly 

 well-adapted, but not so much so as if it occupied the 

 higher peak. Its original position and its variability 

 are shown by the dotted circle. As a result of increased 

 rate of mutation or of reduced selection, or both, the 

 variability of the population has increased and it 

 now spreads down to lower positions on this Mount 

 Minor Adaptation. It contains more aberrant indi- 

 viduals and even freaks than when subject to less 

 frequent mutation or to more severe selection, and a 

 freak may appear that is more adaptive; but this 

 important end has been achieved at the expense of 

 the variability which might have made a major ad- 

 vance possible. 



Figure C introduces a different situation. As a 

 result of environmental change Mount Minor Adap- 

 tation has disappeared and the adapted population 

 has been able to move to a new location at about the 

 same level formerly occupied; now it is on the slope 



