182 SELECTION. 



loss of some types, which occur together in one group. Minor- 

 ities tend to disappear in this way. If we suppose, for the sake 

 of simplicity, that the potentialities of a small group are such, 

 that either one of two types can eventually become predomi- 

 nant and exclusive in the group, what are the factors which 

 actually determine the outcome? 



Other things being equal, the numerical pre-pronderance of 

 one type, or to be more exact of the gametes produced having 

 a certain gene over the number of gametes produced without it, 

 will decide the eventual type. At any moment at which only 

 a few individuals happen to be the parents of the next genera- 

 tion, the chance that the production of one kind of gamete is 

 within the potentialities of these few individuals is greatest for 

 the majority type. But on the other hand, numerical propor- 

 tions being equal, greater or less fitness of constitution for the 

 given environment will decide. Natural selection will in those 

 circumstances become an important factor. 



In those special circumstances, when relatively small groups 

 are living out of touch with the main body of individuals of 

 closely-related species, natural selection must have an import- 

 ance in deciding the outcome of the natural experiment in 

 differentiation, which it cannot have elsewhere. 



We have common species and rare species, and we have 

 species which are common in certain localities and rare in 

 others. This must mean, that their constitution makes it 

 possible for them to live only, where we observe them. 



But we know, that there is a continual striving of seeds and 

 young animals to try life in places, which are not suitable for 

 their constitution. Conditions change, and often we can notice 

 how certain localities are apparently very suitable for certain 

 plants or certain animals without being occupied by those organ- 

 isms. Eventually, chance will bring them there, and it is clear 

 that the less likely this is to happen, the better will be the 

 temporary isolation afforded to the small group. The chances 

 against the evolution of new plants and animals, at least for 

 as far as we are concerned with allogamous plants and animals, 



