68 WEEVIL-RESISTING ADAPTATIONS OF COTTON. 



is believed that the cotton phiiit must first have originated in some 

 measure the protective characters before the external conditions (in 

 this instance, the weevils) conld make them of advantage to the 

 plants and thus encourage their further development. 



The older theory that environment and natural selection are the 

 efficient or actuating causes of evolutionary change has lost many 

 adherents in the last -decade, especially among those who found 

 themselves unable to credit any longer the idea that all the characters 

 and difl'erences of plants and animals are, or have been, of use to 

 them. It has been shown, too, by Professor Weissman and his fol- 

 lowers, that direct adaptations or responses of individual organisms 

 to the environment are seldom or never inheiited by their offspring. 

 To take the place of the doctrine of direct environmental influence 

 in evolution it has been suggested that there may be an internal 

 "' hereditary mechanism," as it has been called, which determines 

 adult characters in advance, in the reproductive cells, so that modifi- 

 cations of the specific or varietal type can arise suddenly. Selection 

 would determine, of course, which of such new "mutations" should 

 survive, but it would be a mere accidental coincidence if the new 

 character happened to fit the conditions better than the old. 



It is possible, however, to explain evolutionary progress and select- 

 ive adaptations without ascribing them either to external causes or 

 to theoretical internal mechanisms. The diversity which plants or 

 animals of the same parentage often show under the same conditions 

 makes it evident that there is no })recise mechanism which determines 

 their form in advance, and all attempts at securing any absolute uni- 

 formity or " fixity " of form and color have failed. The fact is that 

 organisms, even of the same species or variety, are normally diverse, 

 and must have ancestry mixed by interbreeding if bodily vigor is to be 

 maintained for any great number of generations. 



The generalized " specific type," which is a product, as it were, of 

 this diversity and interbreeding, is constantly and gradually chang- 

 ing, and in many ways at once, though in some characters more rap- 

 idly than in others. Selection, ^Yhile in no strict sense a cause of 

 this vital motion of the species or variety, may profoundly influence 

 the direction and rate of change. Selection, in other words, explains 

 adaptation, but does not explain evolution." 



The word adaptation is used in more than one sense by writers on 

 biological subjects. Some treat as adaptations the changes of form 

 or structure by which many i)lants and animals are able to conform 

 to the needs of different conditions. There ai-e several plants, for 

 example, which haxc noiMual broad leaves when they grow on land, 

 and very narroAv and much-divided leaves when they grow submerged 



"Natural Seh-clion in Kinetic Evolution, Science. N. S.. 1!>:."»4!». ItMi-i. 



