IMMEDIATE CAUSES OF MIGKATION. 
107 
shrink or lessen the size of the softer parts. As a very general rule, more 
species of the CEdipoid groups are found in barren areas than of the Acri- 
didae; the Acrididce, on the other hand, are generally fleshier or, to use a 
botanical term, more succulent than the (Edipodae; it follows, therefore, 
that the effect of unusual dryness would manifest itself much sooner in 
the Acridiaus than in the OEdipodeans, and such is undoubtedly the 
fact. Although we are unable to follow out fully the effect of the dry 
condition of the atmosphere on the locust system and tell just how and 
why it results in the migratory habit, yet we are satisfied we are now on 
the direct course toward and very near the solution of the question. 
The usually slender form and unusually hard and horny condition of 
the external crust agree with this theory, as does also the tendency of 
the ultimate sternite of the male (sub-anal plate) to grow narrower and 
become elongated. During the long- continued dry seasons of 1874-187(> 
the effect was very visible on Caloptenus differentialis, a species which 
delights in rank vegetation, and is, if possible, more lubberly than C 
Mvittatus. Examining specimens of a swarm that had flown from Ken- 
tucky and had fallen in the streets of Cairo, 111., in large numbers, I 
was astonished at the change from the typical form ; they had a wild, 
ferocious look similar to that observed in C. spretus ; their bodies were 
much slenderer than usual ; there was a compactness or, to use a term 
which expresses exactly their appearance, a "trimness" of form never 
before observed in them. 
If the theory here advanced is correct — and it is really but a slight 
extension of an opinion long maintained and generally received — we 
have a partial explanation at least of the reason why C. spretus became 
migratory and the (Edipodeans, inhabiting the same region, did not. 
We also have a theory which agrees with all the data bearing upon the 
question and which explains most of the facts and does not stand in 
opposition to any. If this theory is correct, and we are not to suppose 
a species was formed with a migratory instinct, then it follows that 
whatever produced or brought about the arid condition of the area 
where a migrating species originated or became migratory was the ulti- 
mate cause of the migratory instinct. 
As the arid condition of the mountain region and plains of the West 
is, in a geological sense, of comparatively recent date, I think it more 
than probable that C. spretus is but a modified form of C. femur-rubrum ; 
and that several of the apparently local species found in Kansas^ 
Nebraska, Texas, and Minnesota, which are closely allied, are, so to* 
speak, but results of the visits of C. spretus; stragglers, left in their at- 
tempt to return to their native habitats, produce a second brood in lim- 
ited numbers and a combination of favorable circumstances preserve 
them from the usual fate, and in this way the local form results. 
Immediate causes. — If the theory advanced is correct there is no neces- 
sity for searching after immediate causes, as they are essentially or we 
might say constitutionally migratory and will take to flight without any 
