268 THE EFFECT OF MOISTURE 



It may have been noticed that in speaking of these 

 adaptations of terrestrial plants to water, and of 

 aquatic plants to land, it has been more or less tacitly 

 assumed that the effects observed were due to the direct 

 influence of the surroundings on the tissues. It is of 

 course possible that they are partly or even largely in- 

 direct, and that the change of habitat merely calls up 

 latent characters long since possessed by the ancestral 

 plants which lived in similar surroundings. 



Upon members of the Animal Kingdom, observa- 

 tions as to the effect of moisture are exceedingly 

 meagre. This is probably attributable to the fact that 

 in most cases a direct effect is either slight or wanting. 

 Thus Merrifield * could not observe any influence upon 

 the pupae of certain Lepidoptera (E. autumnaria and 

 S. illustraria), nor could Standfuss upon those of cer- 

 tain other species. Koch,f however, came to the con- 

 clusion that a long period of dry or moist weather might 

 exercise a considerable influence on the size of the suc- 

 ceeding generation. Immediately after a continuously 

 dry summer, butterflies are always smaller than after 

 a moist one. Likewise also the second generation of 

 Argynnis selene, which takes flight in the height of sum- 

 mer, is always smaller than the spring generation; but 

 it seems to me highly probable that these effects are of 

 an indirect nature, dependent, perhaps, on changes 

 effected by the moisture in the vegetation on which the 

 larvse feed. 



Leydig t has endeavoured to trace a connection be- 



* Trans. Ent. Soc. 1891, p. 163. 



f Quoted from Elmer's " Organic Evolution," p. 152. 



i " Organic Evolution," p. 97. 



