212 COOK 



The power to make or maintain such adjustments, whether 

 by changes of muscular or other tissues, may well be reckoned 

 as a character of a species, but there is nothing to show that 

 morphological powers of adjustment are different in any evolu- 

 tionary respect from the others, or that they afford any warrant 

 for the inference that evolutionary changes are due to environ- 

 mental differences, or that they arise first as adjustments to 

 external conditions. Any change which increases fitness has 

 the advantage of selective encouragement, and is thus able to 

 exert a larger influence in determining the evolutionary course 

 of the species, so that evolution tends ever toward greater fitness, 

 though other lines of progress are not excluded. If changes 

 could take place only in adaptive characters, the difficulty of 

 maintaining fitness would be greatly increased, because charac- 

 ters would need to be useful from their very inception, whereas 

 they have now the possibility of becoming useful at any stage 

 of their expression. Selection begins to discriminate against a 

 character only when it has become harmful. 



SELECTIVE PERFECTION OF ADAPTATIONS. 



It is not intended to imply that there are never any direct 

 reactions to environmental influences or that such reactions are 

 never of advantage to the organism. The Washingtonia palm 

 of the deserts of Southern California has a complete covering 

 of dead leaves over the whole length of its trunk, and secures, 

 no doubt, a very desirable protection against the extreme heat 

 and dryness. The retention of the leaves is made possible 

 because the climate is dry. Palms native in humid regions 

 usually drop their dead leaves promptly, but if not they are soon 

 weakened by decay and fall away. Such coincidences could 

 scarcely be avoided in any relations so complex as those of 

 biology, but it does not appear that they are of a nature or fre- 

 quency to give them more than a very subsidiary importance in 

 evolution. 



A plant or animal that encounters adverse conditions and is 

 not able to obtain sufficient food will remain stunted. This 

 small size is an advantage, however, in a region where food is 

 scarce or uncertain. Nevertheless it is those individuals of the 



