ASPECTS OF KINETIC EVOLUTION 213 



species which are naturally small, that is smaller than most of 

 their kind, even under favorable conditions, which would be 

 able to make this reaction most successful, since they would be 

 less stunted, or less abnormal, than the others. Thus even the 

 simplest cases of environmental reaction are not to be separated, 

 for evolutionary purposes, from the phenomena of normal diver- 

 sity among the members of the species. Selection, as far as it 

 influences the movement of the species toward adaptation, works 

 through this intraspecific diversity rather than through the 

 environmental reactions. The reactions are not selected, but 

 the individuals which happen to excel in making the reactions. 



Another case illustrating the same principles is that of the 

 inconspicuous colors of the desert animals. Selection is sup- 

 posed to have produced these inconspicuous colors because they 

 conceal the animals, and thus give them protection against the 

 enemies to which they would otherwise be very much exposed. 

 The insecurity of this assumption becomes apparent as soon as 

 we consider the equally striking fact of nature that desert plants 

 also have the same series of dull shades of pale grayish and 

 brownish colors. It would seem, therefore, that evolutionary 

 inferences regarding the colors of the desert organisms will 

 have to provide for the plants as well as for the animals, and 

 that they must not depend wholly upon the idea of protection 

 against predaceous foes. 



From the plants it is very easy to gain another clue to 

 causes of the obscure coloration. The vegetative tissues of 

 desert plants are usually as green as those of species native in 

 humid regions, but in arid climates the soft, thin-walled, green 

 cells have to be covered by thick integuments to protect them 

 from the dry air, and from too great intensity of light and heat. 

 The modified colors seem to be purely incidental to the modified 

 integuments which mask the green tissues within. The thick- 

 ened, specialized outer skins simply protect the plants against 

 the too rapid loss of water, and enable them to withstand more 

 severe conditions of drouth. Many other species living under 

 exactly the same conditions of exposure are nevertheless able to 

 retain the fresh green colors of plants of humid regions, because 

 they have solved their transpiration problems in other ways, just 



