THE STATUS OF MAN. 263 



species. The occasional red and white individuals produced by 

 it constitue a variety. Oenothera biennis is a species. The pale- 

 yellow individuals it sometimes produces are a variety. 

 Varieties can become species, if there is something in their 

 make-up, or in the circumstances which make matings between 

 its members more frequent than matings of its members with 

 individuals of the species. Therefore, in self-fertilizing plants a 

 variety is an incipient species. Self-fertilization saves it from 

 going under into the species. But generally speaking, if we 

 except self-fertilizing organisms, varieties do not become spe- 

 cies. Species arise in a different way, in any way by which a 

 group of organisms is split off from random mating with the 

 whole group to which it belonged. 



The most efficient cause for the reduction of variability in a 

 species is not selection, but the fact that few individuals have 

 a great number of descendants and many individuals have no 

 descendants at all. This reduction of variability is very differ- 

 ently effective in different species. If we know that the num- 

 ber of tapeworms of a given species remains approximately 

 constant from generation to generation, if we further look into 

 the dramatic sequence of lucky, rare coincidences which are 

 together necessary for a single egg to develop into a mature 

 worm, and if we count the millions of eggs produced by one 

 individual, we can see how amazingly rapid the process of 

 specific purification must be in these animals, and how very 

 ineffective occasional crosses must be here. If, on the other 

 hand, we compare the number of off-spring produced by one 

 African elephant to that produced by one tapeworm, if we 

 compare the infant mortality of the two animals, we are im- 

 pressed by the relatively small powers of automatic reduction 

 of variability inside the species, and the relatively great effect 

 of a hypothetical cross in the Elephant. In other words we 

 see, that the barriers that keep apart species in such organisms 

 as the elephants and the tigers must be more effective than the 

 barriers, that keep species of flies and elm- trees and salmon 

 apart. 



