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THE AMEBIC AN NATURALIST [Vol. LIV 



in some groups than in others, to select plants of very 

 specific families or even genera. This must not be under- 

 stood to mean that the individual species of insects affect 

 indiscriminately many or all members of the plant group, 

 but that their normal food-plant or plants do not fall out- 

 side the group. With the exceptions in mind, the fixity 

 of the instinct to feed on only certain kinds of plants is 

 all the more extraordinary, for we cannot readily dismiss 

 it as a physiological or nutritional necessity. 



An interesting light upon the effect of the environment 

 in influencing the selection of food-plants, is furnished 

 by several widely distributed species and genera of but- 

 terflies. Thus several species of Vanessa have quite 

 identical food-habits throughout the entire holarctic re- 

 gion and the same is true of several very closely allied 

 palaearctic and nearctic species belonging to the same 

 group, while, as mentioned above, the general food habits 

 of the larger groups run closely parallel among their rep- 

 resentatives on the two c( nit in ei its. Still more inteivst in, u 1 

 in this respect are the butterflies of the closely related 

 genera Catopsilia and Callidryas which restrict them- 

 selves to the Leguminous genus Cassia. These butter- 

 flies occur in the nearctic, neotropical, Indo-Malayan and 

 Australian regions and such species as have been reared 

 show this preference, which is probably universal. The 

 wi ll known uvnns PajiHin supplies some similar peculi- 

 arities in that several world-wide groups of the genus 

 are restricted to certain closely related groups of plants 

 (e.g., Aristolochia, Citrus, etc.). On the other hand, one 

 North American species, the common Papilio glaucus, is 

 known to affect food-plants belonging to no less than fif- 

 teen different families of plants. "With such constancy 

 in the most remote quarters of the globe among related 

 species of this genus and with one species in a single 

 region regularly developing on the most diversal plants, 

 we must believe that the fixed instincts of some species 

 are not to be led astray by the many temptations offered 

 even by the varied plants of widely separated zoological 



