THE STRUGGLE FOR EXISTENCE. 77 



one man to perish of a disease while another recovers. 

 Nevertheless, selection is of course ready to avail itself of 

 large variations or sports, if they are in the right direction. 

 However, it has been shown above (p. 68) that extreme 

 variations seldom succeed, doubtless because they tend 

 to upset that nice adjustment of parts, that harmony of 

 function so essential in competition. Indeed, we can 

 hardly imagine that the complex adaptations so com- 

 monly found, the marvellous cases of protective resem- 

 blance between organisms and their surroundings, and 

 similar developments, could have arisen otherwise than 

 by the accumulation of small differences step by step. 



For it must never be forgotten that variation is not 

 itself adaptive. Obviously i& it were adaptive, a species 

 would never become extinct, since the right kind of varia- 

 tion would then always be presented to meet the demand. 

 Variation, on the contrary, is blind, fortuitous, but takes 

 place in many directions. Great as may be the number 

 of possible directions, it is not infinite. In a sense, 

 variation is limited ; for all changes in the hereditary 

 constitution are, in the end, due to additions to, subtrac- 

 tions from, or rearrangements in factors already present. 

 Therefore, new developments are greatly influenced by 

 the general hereditary constitution of the race, and, we 

 may add, some sorts of mutation are liable to occur 

 more often than others, and repeatedly in the same or 

 even in different groups (albinism, &c.). 



The next point to consider concerns the usefulness 

 of characters. It is often urged against the theory 

 of natural selection that many characters are useless. 

 Now, we have just maintained that variations are not 

 adaptive they may be useful, harmless, or harmful. 

 But this does not invalidate the Darwinian doctrine ; on 

 the contrary, if variations were always useful selection 

 could not take place. But if it could be shown that a 

 cumulation of harmful or even useless variations had 

 occurred, we should have a serious difficulty to deal with. 



