EXTENSIONS OF DARWINISM 259 



as in rapidity, from any of nature's changes during the whole 

 period of the development of existing species. 



But although I feel confident that the known amount 

 of variation would amply suffice for the adaptation of any 

 dominant species to a normally changing environment, I 

 admit that there are conceivable cases in which changes 

 may have been so great and so comparatively rapid as to 

 endanger the existence even of some of those species which 

 had attained to a dominant position ; such, for instance, as 

 the opening of a land passage for very powerful new Car- 

 nivora into another continent or extensive area (as appears 

 to have occurred with Africa in Tertiary times), in which 

 case it is quite possible that such an animal as the American 

 bison might have been first reduced in numbers, and, for 

 want of any sufficiently rapid development of new means of 

 protection, be ultimately destroyed. 



But a few years ago an idea occurred independently to 

 three biologists, of a self-acting principle in nature which 

 would be of such assistance to any species in danger of 

 extermination as, in some cases at all events, would enable 

 it to become adapted to the new conditions. It would, 

 in fact, increase the powers of natural selection, as above 

 explained, to a degree which might sometimes make all the 

 difference between life and death to a certain number of 

 species. It depends upon the well-known fact that the use 

 of any limb or organ strengthens or increases the growth of 

 that part or organ. On this fact depends all training for 

 athletics or games ; and it is alleged by some trainers that 

 any one, however weak naturally, can have his strength 

 very greatly increased by systematic but carefully graded 

 exercise. If, therefore, the survival of any animal in presence 

 of a new enemy or unaccustomed danger depends upon in- 

 creased powers of running, or jumping, or tree-climbing, or 

 swimming, then, during the process of eliminating those 

 individuals who were the worst in these respects, all the 

 remainder would have to exercise their powers to the utmost, 

 and would, in the act of doing so, increase their power of 

 escaping the danger. Thus a considerable number would 

 become capable of surviving, year after year, to a normal 

 old age, and during this whole period would, year by year, 



