152 Proceedings of the Boyal Physical Society. 



It is extremely probable that during the evolution of the 

 elephants, incipient varieties, modified in the same direction, 

 at the same time, and in the same area, sometimes escaped 

 the swamping effects of intercrossing by wandering from the 

 ancestral home, or by having a preference for each other, by 

 being more or less infertile with the parent species and other 

 varieties, or by flourishing under conditions repugnant to or 

 unsuitable for other varieties. 



But while any or all of these may have assisted in effecting 

 alterations in the mandible and trunk, the teeth and cranium, 

 I believe new varieties were able to establish themselves 

 mainly because of some unusually favourable condition of 

 the germ-plasm — subtle alterations, neither capable of being 

 seen nor measured, which resulted in increased vigour, in 

 exclusiveness, or in the power to so completely dominate 

 or control the development, with the result that such 

 fortunate varieties not only escape being swamped by inter- 

 crossing, but either swamp the parent form and other 

 varieties they happen to encounter, or prove so exclusive 

 that the offspring faithfully reproduce the one variety or 

 the other. 



If it is admitted that Germinal Selection finishes the work 

 begun by the environment — finally decides which varieties 

 are to continue, which to disappear, I think we may agree 

 with Wallace that Natural Selection is the sole factor in 

 organic evolution. 



