REDUCTION OF VARIABILITY. 135 



counted as a minority in a mixture of two forms, subjected to 

 the same factors which govern the number of individuals 

 having off-spring in every generation. 



If an individual or a few individuals found a colony, stock a 

 field, the colony may become numerically of sufl&cient import- 

 ance to persist. In groups of plants where selfi-ertilization or 

 asexual propagation is the rule, but where nevertheless^ occa- 

 sional crossing is not excluded absolutely, ntimerous new spe- 

 cies, of different biotypes may coexist in the same ecological 

 niche. In such cases we understand that continually some of 

 these species fade into insignificance and disappear, whereas 

 new species are continually produced. We may feel sure that 

 wherever we meet with such a group of species, such a poly- 

 morphic group, we will find that either asexual multipUcation 

 or self-fertilization is the common mode of reproduction. Wheat, 

 barley, Viola tricolor are typical examples of self-f ertiUzing poh- 

 morphic plants, and the dandelion and stinging-nettle are good 

 examples of polsmiorphic groups of plants where asexual mul- 

 tiplication is common. Real polymorphy, the exi'stence of nu- 

 merous different relatively pure species in the same ecological 

 niche is only possible in such plants. Pol5nnorphy in animals is 

 possible only under the s&me conditions, in imi-cellular organ- 

 isms, in vegetatively reproducing animals, and in instances of 

 isolation within the species through sexual selection. The cases 

 of the persistence of black individuals in moths and certain 

 birds, pointed out by Bateson in his book on the problems of 

 Genetics may fall into this category. 



We have seen how it is possible, that occasionally individ- 

 uals are produced in populations of allogamous animals and 

 plants, which differ sufficiently physiologically to come to fit a 

 different environment, in which they would tend to be a new 

 species. 



This mode of origin of a species must be common to the free- 

 crossing and the self-fertilizing orgamsms. We may caU it phys- 

 iological isolation. Any kind of isolation must tend to species- 

 formation, to the production of new groups, having their own 



