348 THE AMERICAN NATURALIST [Vol. XLVI 



89,401 : 90,298 : 22,801 = 11.2 per cent, recessive. 



The change in any case is not so simple as mathemat- 

 ical study would at first suggest, for many of the indi- 

 viduals mate with members of earlier or later genera- 

 tions. When a considerable number of recessive mutants 

 appear in a given generation this excess is, in part, re- 

 duced by matings with members of the normal genera- 

 tions before and after them. Hence a return to the nor- 

 mal or usual condition is made easy. These considera- 

 tions naturally suggest a reason for the comparative 

 stability of species within a genus or of elementary 

 species within a Linnsean species. 



From what has been said it will be seen that unless a 

 recessive character is arising by mutation at some ap- 

 preciable rate there will be little or no increase in the 

 proportion of individuals exhibiting this character. On 

 the other hand, whenever a considerable number of mu- 

 tants do appear the normal condition of equilibrium is 

 disturbed and a new stable ratio becomes established in 

 which the recessives are in larger proportion. 



There are probably many recessive characters which 

 are not the objects of natural selection by the environ- 

 ment nor of sexual selection in mating. Lefthandedness 

 may well be such a character. It certainly seems that 

 the proportion of individuals with such characters will 

 increase slowly through long periods of time. Let it be 

 supposed, for example, that the proportion of dominants, 

 heterozygotes and recessives in the general population 

 is 9 : 6 : 1. Now it is probably far easier in mutation for 

 a given factor to drop out than to be added. If, however, 

 the dropping out and addition could take place with 



