372 
The Colorado Potato-Beetle. 
to North America : and it must be repeated, that no North 
American beetle has yet naturalized itself on this side of the 
Atlantic. 
A more valid argument may be founded on the evident special 
adaptation of the potato-beetle, and the group of species to which 
it belongs, to the region in which they have been hitherto 
confined. In the natural distribution of the genus, the group is 
quite unknown beyond a circumscribed area, and that area 
marked by peculiar continental conditions of climate, namely, 
the interior of the North American Continent. It is well known 
to naturalists that the species of all large genera fall into minor 
groups, closely allied in their organization, and always strictly 
confined to definite minor geographical areas. No explanation 
can be given of this law other than that the groups so formed have 
become specialised to a high degree, with close reference to the 
conditions of the area which they inhabit. Such species are 
never cosmopolitan wanderers, like those of genera which show a 
looser adaptation to their locale. It will readily be granted by 
entomologists that the potato-beetle group shows this restricted 
adaptation in a remarkable manner. Two out of three of its closely 
allied species belong to elevated plateaux in the interior of the 
continent, and tend in their range towards the tropics rather 
than towards the north. The potato-beetle itself is found as far 
South as the city of Mexico, at an altitude of 5000 feet , and its 
nearest relative (a species with difficulty distinguishable from it), 
the Doryphora W-lineata, is confined to Mexico, and the country 
next to the South — Guatemala. A third species, D. Jniicta, is 
known only in the middle States, from Georgia to Southern 
Missouri. No species at all approaching this group in natural 
affinity is found inhabiting Europe, or any part of the world 
other than the warmer temperate and tropical regions of the 
American Continent. It is quite otherwise with the two 
European beetles of the same tribe which have naturalised them- 
selves in the United States, viz., the asparagus-beetle and the 
elm-leaf eater, the latter having numerous North ^ American 
native relatives, and the former belonging to a group which 
ranges over the world, though curiously enough represented in 
North America by Mexican species only. If it be objected that 
the potato-beetle, though originally restricted to a peculiar region, 
has departed from the habits of its group, and developed a 
powerful migratory instinct, the answer is that in spreading it 
has kept very closely to a tract of country possessing a similar 
climate to that of its native home — the climate of the States in- 
vaded being characterised by the same hot fine summer, and cold 
dry winter, during which latter the ground is always protected 
by a coating of snow, an essential condition to a creature which 
