ASPECTS OF KINETIC EVOLUTION 205 



animals and sew their pelts together. We say, similarly, that 

 a desert climate "causes" a plant to become more hairy, but 

 this is as yet a mere figment of speech. We have no notion of 

 the chain of biological events coming between the dryness and 

 the hairs. We can appreciate the advantage of the reduced 

 transpiration, but we do not know how the plant puts on the 

 additional protection against the dry atmosphere. 



ALTERNATIVE ADJUSTMENT CHARACTERS. 



We shall hardly come to understand aright the relation of 

 fitness to evolution until we accustom ourselves to thinking of 

 these variations of accommodation or so-called " environmental 

 reactions " as expressions of the power of the plant or animal to 

 choose, as it were, between alternative methods of growing and 

 of conducting the functions of existence. 



Organic versatility, plasticity, or whatever it may be called, 

 does not conduce to the rapid development of specialized char- 

 acters (adaptation), or to the multiplication of new groups (specia- 

 tion), but it is undoubtedly of vast practical importance in the 

 economy of species. Some species have little of this readiness 

 of adjustment, while others are able to adopt a great variety of 

 forms and can thus take advantage of opportunities of existence 

 under a great diversity of natural conditions. By keeping open 

 a larger number of alternative lines of progress, the power of 

 accommodation very greatly increases the ability of species to 

 solve their environmental problems. The environment is unable 

 to prevent such groups from accumulating many kinds of varia- 

 tions or from making trial of them, as it were, in a great variety 

 of combinations. This affords the best of opportunities for the 

 construction of new types with enlarged environmental resources, 

 instead of providing merely for the differentiation of narrowly 

 localized and specialized species. 



The different characters assumed by a species in accommodat- 

 ing itself to different environments are not less characters of the 

 species because they are shown simultaneously than if they 

 were developed in successive epochs of evolution. The only 

 sense in which they are not characters of the species is the nar- 

 rowly taxonomic one in which species are treated as having 



