SELECTION. 



Elsewhere we have discussed the different factors in evolu- 

 tion, and the circumstance, that on the whole those factors can 

 be grouped in two categories, factors which heighten the poten- 

 tial variability of groups, and such as reduce the potential 

 variability. 



In regard to species-formation, we can say that those causes 

 which heighten the Total potential variability of a group, 

 produce the conditions required for the possibility of the origin 

 of new species; and that those causes which reduce the Poten- 

 tial variability, will make a species of any group of organisms 

 under certain conditions of relative isolation. 



How does selection stand in this respect, to which category 

 must we bring it? Is selection always a cause or reduction of 

 the Total potential variability, and thus a factor of some 

 importance in the establishment of species? 



Darwin, and later Weismann tended to see the cause of 

 specific purity and the reason for its continuance in natural 

 selection. Natural selection was thought to keep a group of 

 organisms with a natural tendency to vary in all directions 

 down to a limited variability, by a weeding-out process, which 

 would tend to conserve only the individuals which were best 

 adapted to the conditions under which the species lived. 



We have tried to show, that a group of organisms which is 

 limited in certain ways, which is so situated that the propor- 

 tion of matings with individuals from outside remains below 

 a certain maximum, as compared to matings between its mem- 

 bers, will automatically reduce its potential variability. It is a 

 matter of relation between the influence of factors heightening 

 the variability (crossing) and factors reducing it, (isolation, 



