Geographical Distribution of Insects. 99 



existence is so inseparably connected with that of a particular 

 species of plant, that if the latter become extinct the former 

 must necessarily perish along with it. Neither has it been 

 shewn that any of them follow a vegetable species through- 

 out the whole extent of its habitat, the one existing wherever 

 the other exists ; but it is certain that many natural groups of 

 iiisects are connected with corresponding botanical groups in 

 respect to nom'ishment It is thus that the genus Papilio, 

 very numerous in species, and subdivisible into a great num- 

 ber of secondary gi'oups, contains, among the latter, some 

 which Uve only on the citrus, others on the umbelliferae, 

 laurel, sassafras, &c. — whence result many consequences, 

 some of them of direct interest in a botanical point of view. 

 Thus, 



1. A plant happening to disappear from a locality, the spe- 

 cies of insect which it noiu-ished may have recourse to an allied 

 plant of the same family, and thus maintain itself in the lo- 

 cality in question. 



2. When a plant in one country supports a certain species 

 of insect, if we happen to discover, in a very distant country, 

 another plant of the same group, we may often conclude, a 

 ])riori, that this country likewise possesses an insect of the 

 same genus as the other. The numerous species of the Le- 

 pidopterous genus Lyhithea, for example, live on the Celtis in 

 the caterpillar state, and there is one species in the central 

 districts of France which feeds on the leaves of Celtis australis. 

 The genus Celtis Ukewise occvirs in the Antilles, Madagascar, 

 and Java, and in each of these countries a particular species 

 of Lybithea has been found. It will be observed, that in such 

 cases the existence of the plant may be inferred from the 

 presence of the insect, as well as that of the insect from the 

 plant, but the former conclusion is by no means attended with 

 the same certainty as the latter. 



3. When a plant is transported into a foreign country, when 

 it finds no congeners among the indigenous vegetables, the 

 insects of the country to which it has been carried do not at- 

 tack it. ThiLS, our cabbages, carrots, vine, &c. acclimated in 

 Cayenne, suffer no injury from the insects of that country ; 

 and in like manner the Indian chestnut, the tulip-tree, and 

 the magnolia, are respected by ours, as well as the greater part 



