Geographical Distribution of Insects. 109 



certain conditions which we know nothing of, and which we 

 are ignorant where to look for. 



It is easy to demonstrate that the causes above mentioned 

 are of no account in producing the existing diversity in the 

 habitations. In regard to that, one of two things must have 

 taken place ; either the species have been created simultane- 

 ously on diflFerent points of the globe, in which case the ques- 

 tion resolves itself ; or else, as the gcnerahty of naturalists 

 formerly thought, they were created at one point whence 

 they were spread over the rest of the globe. But if this had 

 been the case, the species must have been otherwise distri- 

 buted than ^^•e now see them ; they would have been spread 

 in the direction of the isothermal lines, and the same species 

 would be found encircling the globe, following the flexuosities 

 of these lines ; if they became gradually modified as they re- 

 ceded from the point of departure, they would follow, step by 

 step, these successive modifications. But we witness nothing 

 of this kind, and consequently it is more than probable that, 

 at the origin of things, Providence placed these animals in 

 the places where we now see them, adapting each to the cli- 

 mate under which it was destined to live, and conferring up- 

 on it at the same time a sufficiently flexible organization to 

 admit of its departing more or less from the centre of crea- 

 tion. Physical conditions would only have the eff'ect of mo- 

 difying slightly this primitive distribution. 



Another question remains to be discussed, intimately con- 

 nected with the preceding, namely, whether we ought to con- 

 sider as derived from the same stock, individuals of certain 

 species which are spread over the greater part of the earth, 

 or which exist in countries widely remote from each other. 



When two countries, however distant, are in communica- 

 tion by means of intermediate countries, and the insect spe- 

 cies phytophagous, and of powerful flight, we may suppose 

 that it has advanced insensibly into these countries, wherever 

 it found the plant suited to it. It is thus that we can ex- 

 plain the dispersion of Venussa cardai through Europe, Asia, 

 Africa, and New Holland, because, in the caterpillar state, it 

 lives on the carduaceae, malvacea?, urticeae, &c. families of 

 plants which have all representatives in these countries. And 



