2 66 Darwin, and after Darwin. 



Tennyson thus noted the fact, and a few years later 

 Darwin supplied the explanation. 



But of course in many, if not in the majority of 

 cases, anything that adds to the life-sustaining power 

 of the single life thereby ministers also to the life- 

 sustaining power of the type ; and thus we can under- 

 stand why all mechanisms and instincts which minister 

 to the single life have been developed — namely, 

 because the life of the species is made up of the lives 

 of all its constituent individuals. It is only where 

 the interests of the one clash with tliose of the other 

 that natural selection works against the individual. 

 So long as the interests are coincident, it works in 

 favour of both. 



Natural selection, then, is a theory which seeks to 

 explain by natural causes the occurrence of every kind 

 of adaptation which is to be met with in organic 

 nature, on the assumption that adaptations of every 

 kind have primary reference to the preservation of 

 species, and therefore also, as a general rule, to the 

 preservation of their constituent individuals. And 

 from this it follows that where it is for the benefit of a 

 species to change its type, natural selection will effect 

 that change, thus leading to a specific transmutation, 

 or the evolution of a new species. In such cases 

 the old species may or may not become extinct. If 

 the transmutation affects the species as a whole, or 

 throughout its entire range, of course that particular 

 type becomes extinct, although it does so by becoming 

 changed into a still more suitable type in the course 

 of successive generations. If, on the other hand, 

 the transmutation affects only a part of the original 

 species, or not throughout its entire range, then the 



