I ADJUSTMENT, DIRECT AND INDIRECT 

 lere idiosyncrasy, such as we call fortuitous, or 

 Ihether it is one that is manifested simultane- 

 ously by a large number of individuals, so that 

 ,^^ may be traced to causes acting upon them all 

 '^Bi common. Now this latter case is the one 

 ^Hhich must here be taken into the account. If 

 ^^ large number of individuals may simultane- 

 ously vary in a given direction, and if this may 

 often happen within the limits of single genera- 

 tions, it is obvious that we have here a factor of 

 specific change not to be lightly passed over. In 

 estimating the effects of natural selection upon 

 a number of variations which are, quite legiti- 

 mately, taken for granted, we must not forget 

 to generalize the variations in connection with 

 some common cause to which they may be 

 assignable. Now it cannot be denied that in 

 any single generation of organisms variations 

 are very likely to occur, throughout nearly the 

 whole number of individuals, which are due to 

 the direct adaptation of the species to its envi- 

 roning circumstances. When exhibited in the 

 effects wrought upon the human constitution by 

 exposure to changed physical conditions, such 

 variations are known as acclimatization. Within 

 the infinitesimal period of two centuries the 

 English race in America has come to differ per- 

 ceptibly, though very slightly, from the Eng- 

 lish race in Europe ; and this very slight differ- 

 ence, which cannot be explained by the much 

 79 



