16 Darwinism and Other Essays. 



Operating in this stern way, natural selection 

 secures the general adaptation of each race of or- 

 ganisms to the conditions of life which surround 

 it. And so long as a species continues surrounded 

 by circumstances that are tolerably persistent, 

 natural selection maintains its stability of char- 

 acter. Thus what the older naturalists called the 

 " fixity of species " is fully accounted for. But a 

 " fixity of species " that is maintained only under 

 such conditions is really no fixity at all. Change 

 the surrounding circumstances, and the average 

 character of the species must change. Slight pe- 

 culiarities that once insured survival will now 

 insure destruction, and tendencies to vary that 

 once would have been nipped short will now be 

 encouraged and exaggerated. In this way the 

 strong tendency, hereditary in all mammals, to- 

 ward the growth of hair on the surface, was 

 greatly exaggerated in the Siberian mammoth, 

 while checked in his brethren, the elephants of 

 India and Africa. In this way a peculiar curve 

 in the contour of butterflies' wings, which is per- 

 sistently killed out in India and Java, is with 

 equal persistency selected for preservation in 

 Celebes. How far such alterations in the direc- 

 tion of natural selection may work deep-seated 

 changes in the structure of an organism one can- 



