OCCASIONAL ABUNDANCE OF INSECTS. 29 
selves not only admit but rest half their theory on namely 
that these venturesome individuals, who according to them 
do cross the narrow seas, leave offspring only to be exter- 
minated by the rigor of a climate unsuited to their 
constitution, that is to say that the emigrants leave no 
descendants, those who stay behind do—and yet the 
tendency to emigrate becomes confirmed and perpetuated 
in the race—and this is survival of the fittest ! 
Now so constant, so inflexible seems to be the application 
of this great principle, that only the fittest survive, that 
this one consideration does for me effectually dispose of 
the migration theory of the erratic abundance of Deilephia 
galu. That is, that allowing such a migratory instinct, 
or tendency to dispersal—call 1t what you hke—ever to 
have arisen, either such a tendency would have become 
eliminated, or the whole species would have become 
extinct, ifits exercise led as ex hypothest we must believe 
that it would lead to the utter extermination of these in- 
dividuals who practise it. That conclusion seems to me 
unavoidable, and I would so leave that part of the subject, 
holding that neither by fact nor by theory can the phen- 
omenal abundance of this moth on certain years be 
explained as due to any form of voluntary migration. 
So far then I have treated of what might be called 
voluntary movements, but there are also involuntary 
journeys, and our theorists never seem quite able to make 
_ up their minds as to which sort they attribute the presence 
here in its years of this moth. Involuntary movements 
are certainly much the simpler, there is but one possible 
agent, the wind, and the wind to be a possible agent 
must blow at the required time from the.poimt whence 
the insects are supposed to be driven to the point 
where they are subsequently discovered, and of course only 
imagines can be the subjects of such carriage. Now take 
