OCCASIONAL ABUNDANCE OF INSECTS. 25 
although our friends have not as far as I have seen, in any 
place explicitely defined what they mean by migration as 
applied to this insect, yet it is necessary for us to be clear 
on the point if we attempt to show how in our view no 
kind of migration at all will fit the case. 
You may if you like call that merely indefinite pro- 
eression such as we see in the Locust swarms, in the 
march of the Termites, in the discursive flight of such a 
butterfly as Vanessa cardut, migration; but more justly 
should that term be apphed to the recurrent flight of the 
swallow and the fieldfare. Migration properly so called 
seems defineable as a recurrent change of habitat, definite 
as regards direction, periodic as regards time, induced by 
instinct and originating in some consequential benefit 
to the race. 
Now I think I am safe in asserting that such a migratory 
instinct or practice as this is quite unknown among 
insects. In the first place among the vertebrates the com- 
pelling factor is the need for similar or at any rate 
sufficient food during differing seasons, as well as the 
necessity involved in the correlation of certain organisms 
to certain temperatures, but the general brevity of all 
perfected insect life, and the fact that such forms as do 
survive one summer invariably pass the winter in a more 
or less dormant state, forbid us to believe that among 
them any such instinct could have similarly arisen; and 
in the second place, the entirely dissimilar condition under 
which the various stages of insect development are passed 
make it difficult to imagine how a habit such as definite 
and regular migration could have arisen, which ex 
hypothest must have been a benefit to the insect during 
the whole of its varied existence, as an environment bene- 
ficial during one phase might be quite the reverse during 
another. The migrationists I admit although not very 
