MODIFICATION OF SPECIES 363 



enhanced usefulness. Such changes would necessarily be slow; hence the 

 alteration of the environment would have to be at least semipermanent to 

 accomplish any important modification of the species exposed to it. 

 Even a hundred thousand generations might well prove too short for 

 any important change of a species under the selective action of the 

 environment. 



Among groups of organisms that differ in more than the mere propor- 

 tion of certain genes, selection should work more effectively. Of the 

 varieties that arise by chance within a species, one or more may well fit the 

 environmental situation better than others. Unless these varieties are 

 kept in their separate areas by different physiological responses to features 

 of the environment, which is probably not often true, the favored variety 

 or varieties should gain the ascendancy. They might or might not 

 crowd out their fellow varieties ; but even if they were only more abundant 

 in individuals, they should have a greater share in determining any later 

 evolutionary changes. In like manner, species must differ in their 

 capacity to propagate, and the more capable ones should increase in 

 numbers. Genera, families, orders, all higher ranks must be subject to 

 this action of natural selection, but the action is always on the individuals 

 that compose them. 



Adaptation. — The guidance of evolution by natural selection should 

 result in a considerable degree of fitness for the environment. If individ- 

 uals and species are preserved in proportion to their ability to succeed, 

 their success should grow with the passage of generations. The fact that 

 natural selection offers a general explanation of adaptation is one of the 

 chief reasons for the rapid acceptance of Darwin's theory among biol- 

 ogists. For adaptation is very widespread, and some of it is very remark- 

 able. So abundant is it, and so marvelous are parts of it, that many 

 naturalists have come to feel that adaptation is the outstanding feature 

 of life requiring an explanation. 



It would be easy, however, to overemphasize its frequency, its degree, 

 and its necessity. Most species are not so well adapted to their situ- 

 ation as they conceivably could be, but they get along. Lack of satis- 

 factory adaptation in certain species or larger groups seems to be proved 

 by extinction. Moreover, obvious adaptation, as among taxonomic 

 groups, is found most markedly in the groups of higher rank. Classes, 

 orders, families are marked off from one another by such things as wings, 

 gills, armor plate, webbed feet, and quills which perform definite functions 

 in the lives of the individuals and often help to determine where they shall 

 live. Such structures are highly adaptive. In lower ranks, however, 

 this adaptiveness is much less common. Most genera of the same family 

 do not make any particular use of the characters that distinguish them 

 from one another, though there are exception?. Among species of the 



