ri2 PROBLEMS OF EVOLUTION 



conditions of life were exactly similar, then, if passage from 

 island to island were impossible, we have good reason to believe 

 that differences would arise, and that what might rank as separate 

 species might be formed. But the matter would end with mere 

 diversity, probably representing little more than the stereotyping 

 of the inconstant varieties found within the species before it was 

 distributed among the islands. 



But this limitation to the power of isolation must not blind us 

 to its importance. Natural Selection unaided can only raise a 

 species to a higher level, it cannot increase the number of species 

 it can account, as it has been put, for monotypic, but not for poly- 

 typic evolution. The elimination of the unfit will leave only the 

 fit upon the earth ; hence there will be advance but no diversity. 

 If from one species two or more are to be formed, we must 

 imagine that in some individuals a variation arises that adapts 

 them to an alternative environment that is at hand: birds, though 

 members of a grub-eating species, may find themselves possessed 

 of a strong muscular stomach or gizzard, fitting them to some 

 extent for a diet of grain. If these individuals can somehow be 

 kept apart from the rest to prevent intercrossing with its swamp- 

 ing effects, a new species may be formed. 



Natural Selection and isolation are mutually dependent. 

 Neither could have done much without the other. 



VI 



SYSTEM BY WHICH WASTE IS REDUCED 



indis- The survival of the fittest in the struggle for existence is 

 - certain ly ^ e principle to which the vigour of animals and plants 

 tion is due, and is, I believe, the principle on which we must account 

 for evolution. But we cannot expect the whole truth to be 

 summed up completely and exactly in one short phrase. The 

 question obtrudes itself, Are those that are eliminated always 

 the unfit ? Do not young oak-trees die by the million every 

 year in England simply for want of room to grow, the most 



