EXTEXSIO.\S OF DAKWJMSM l>70 



tinent or extensive area (as appears to have occurred with 

 Africa in Tertiary times), in ^vllicll case it is quite possible 

 that such an animal as the American bison mii^ht have been 

 first reduced in numbers, and, for want of any suflicieutly 

 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 jorinciple 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 explainc(l, to a degree 

 which might sometimes make all the difference between life 

 and death to a certain number of species. It depends upon 

 the w^ell-known fact that the use of any limb or organ strength- 

 ens 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 increased powers of running, or jumping, 

 or tree-climbing, or swimming, then, during the process of 

 eliminating those individuals who were the worst in these re- 

 spects, 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 con-idrrable 

 number would become capable of surviving, year after year, to 

 a normal old age, and during this whole period would, year 

 by year, have fresh descendants, and of these only the very 

 best, the most gifted naturally, would survive. The in- 

 creased adaptation during the life of the individual would not 

 be transmitted, but the quality of being inijirovablc during life 

 would be transmitted, and thus additional tim*^ and a consider- 

 ablv increased ]K>])ulation would give more uuUcrials for 

 natural selection to act upon. With this help the species 



