CAUSES OF MIGRATION. 
103 
planation as the most satisfactory. Our native species, instead of seek- 
ing forests as a place of shelter, appear to avoid them as far as possi- 
ble. 
Darwin's theory is undoubtedly contradicted by the characteristics 
of all the migratory species. 
8. THE CAUSES OF MIGRATION. 
As stated in our First Eeport, 206 "we must recognize the fact that the 
influences bearing on migration fall into two distinct categories, viz, 
remote or general, and immediate or special." We shall therefore briefly 
consider the two classes separately. 207 
a. Remote causes. — Why one species of insect should at certain times 
develop in immense numbers and migrate, while another closely allied 
species, inhabiting the same locality, and to all appearance subject to 
the same influences, never increases to the same extent and never exhib- 
its a disposition to migrate, is a question which has long puzzled ento- 
mologists. 
If entomologists are asked why locusts migrate, ninety-nine out of 
every hundred will probably answer, " On account of excessive numbers? 
We might press the inquiry farther and ask why they develop in such 
excessive numbers. But we propose, at present, to seek for the remote 
causes by another road, as the attempt to press back the inquiry step 
by step would lead us into a labyrinth of biological questions which we 
have no occasion to enter upon at this time. 
A thorough and exhaustive examination of this question would carry 
us back Into the last geological changes of the earth's surface ; but this 
we have not the time to undertake if we felt qualified to do so, which 
we by no means claim. 
As we have heretofore shown, migratory locusts are found only in tree- 
less, dry, and more or less barren regions, and, as a very general rule, 
their breeding grounds are in areas or plateaus of considerable eleva- 
tion*. The native home, for example, of P. migratorius, appears to have 
been the steppes of Southern Siberia and Tartary. A. peregrinum has 
its points of greatest development in Central Arabia and the dry, ele- 
vated table-lauds of Northern Africa ; A. paranense, in the higher bar- 
ren plateaus of Argentine Eepublic; and C. spretus, in the high, barren 
regions of the Eocky Mountaius, and the elevated plains of Montana, 
Western Dakota, and British America. The marked characteristics of 
all these regions are, absence of forests, more than ordinary dryness, and 
rarefied air. Keferstein remarks that — 
A dry, warm, uncultivated, treeless plain, -where the brood can be deposited un- 
disturbed and left to ;;rnw up, is especially favorable to the propagation of the locust, 
and in such districts of country the locust plague appears most frequently and regu- 
m Pago 240. 
^'It is proper to state here that while the Commissioners agree in the main as to the causes of migra 
tion the writer of this chapter must be held alone responsible lor some of the views advanced here. 
