2 18
of the agent of selection, Natural Selection would not come
into play at all. Or, on the other hand, a species of in
sect-eater, which lived on one particular species of victim,
might prefer to eat the slightly nauseous victim instead
of attempting the radical change of seeking a new source
of food altogether. It is natural to suppose that it would
attempt the former course, seeing that it must have studied
the habits of its victims and could catch them more easily
than others. It has been ascertained that certain species
do restrict themselves to one particular victim. Thus
Mr. Bates tells us :—
“We have proof, in the case of sand-wasps, which provision their
nests with insects, that a single species is very generally selected out
of numbers, even of the same genus, existing in the same locality. I
was quite convinced in the case of Cerceris binodis of South America,
which destroys numbers of a Megalostomis (family Clythridce), that
the great rarity of the beetle was owing to its serving as prey to the
Cerceris.”—(Transactions of the Linnean Society, vol. xxiii., pari j,
P• 5H-)
In this case all would be alike eaten, and there would be
no Natural Selection.
Let us now take the alternative explanation ; that an
edible form first began to develop a slightly nauseous taste
in some individuals which still remained unchanged in
outward form. In that case, they could not be dis
tinguished from their unmodified companions. Two very
opposite results have been conjectured as ensuing from
this fact. Mr. Wallace asserts that the modified insects
would be attacked just as much as ever.
“ If any particular butterfly of an eatable group acquired the
disagreeable taste of the Heliconias, while it retained the charac
teristic form and colouring of its own group, this would really be of
no use to it whatever ; for the birds would go on catching it among
its eatable allies (compared with which it would rarely occur), it
would be wounded and disabled even if rejected, and its increase
would thus be as effectually checked as if it were devoured.”—
{Contributions, p. 81.)