REDUCTION OF VARIABILITY. 105 



On one hand we see causes which make the total potential 

 variability of the groups larger, and on the other hand we 

 find things which tend to reduce it, cut it in half or even eli- 

 minate it. 



The potential variability of a group of organisms becomes 

 larger if into this group individuals are taken up, which either 

 possess a gene or genes not heretofore present in any member 

 of the group, or which on the other hand lack genes, common 

 property of all the members of the group. Here of course we 

 see the difference between variability and potential variabili- 

 ty, because the last is concerned with all genes, be they fact- 

 ors in the development or not. Two animals or groups of ani- 

 mals may have the same characters, and the same variability, 

 and yet they may differ in possession or lack of one or more 

 genes, which in these types have no effect. In such a case their 

 hybrid off-spring may not be more variable, and at least in one 

 generation variability may not be increased. The potential 

 variability however, did increase, and this may come to light 

 in the second generation, or even much later in the behaviour 

 of some descendants off-spring from yet another cross. 



We saw that mutation, which means loss-mutation, can at 

 the most play only a very insignificant role either in the height- 

 ening of the potential variability, or in its decrease. With this 

 insignificant exception then, we can focus all our attention 

 upon crossing in the widest sense as the cause of variability. 

 And in this chapter we are concerned with the second part of 

 the evolution question, what causes reduced variability? 



This was the question with which Darwin was mainly con- 

 cerned. Given a certain variability, how do species finally lose 

 this variability and become pure? We know that the answer of 

 Darwin was, that selection causes this reduction of variability. 

 As there will in every group be individuals which are better 

 fitted than others to survive, and as it will be these individuals 

 which on the whole survive, the whole group will gradually 

 tend to change in this selected direction. 



In so far as variation brings about a greater or lesser useful- 



