670 The Rocky Mountain Locust. [ November, 
will they be prompted to migrate, and the farther will they push 
on to the east and south,” 1 
We have, second, the return migration toward the northwest 
from the country south and east of the lines already indicated, of 
the progeny of invading swarms, as soon as wings are acquired 
the next summer. ‘Time will not permit me to present the ex- 
planation of this return migration. In the work just quoted I 
have discussed its causes, the reasons why the species cannot per- 
manently thrive in the Mississippi valley, and the conditions 
which prevent its establishment there. 
We have, third, the eastern limit of the insects’ spread along a 
line broadly indicated by the 94th meridian, and the consequent 
security from serious injury east of that line. 
These three features of our disastrous swarms — the return 
migration from the southeast country (which implies only tem- 
porary injury therein), and the eastern limit, — may be stated 
as laws governing the insect east of the Rocky Mountains. They 
have constantly been urged by me, and the present year’s expe- 
rience has confirmed and verified them. I think I may safely 
present a fourth, namely, that the eggs are never laid thickly 
two successive years in the same regions. 
In mapping out the country in Kansas and Missouri in which 
eggs had been laid most thickly in 1876, I was struck with the 
fact that the very counties in which the young insects had been 
most numerous and disastrous in 1875 were passed by or avoided, 
and had no eggs of any consequence laid in them in 1876. The 
fact was all the more obvious because the insects did much dam- 
age to fall wheat, and laid eggs all around those counties, to the 
north and-south and west. From the exhaustive report on the 
insect in Minnesota, made by Mr. Allen Whitman, it was also 
very obvious that those portions of that State which had been 
most thickly supplied with eggs in 1875, and most injured by 
the young insects in 1876, were the freest from eggs laid by the 
late swarms of the latter year, notwithstanding counties all 
around them were thickly supplied. I was at first inclined to 
look upon these facts as singular coincidences only ; but instances 
have multiplied. A remarkable one has been furnished me by 
Gov. A. Morris, of the northwest territory. You are well 
aware that in 1875 the locusts hatched out in immense numbers 
and utterly destroyed the crops in the province of Manitoba. 
Now, in 1876 they were very numerous over all the third prame 
1 Locust or Grasshopper Plague, page 57. Rand, McNally, & Co., Chicago. 
