Mimicry in Butterflies 
2. That the imitators are always the 
more defenseless. 
3. That the imitators are always less 
numerous in individuals. 
4. That the imitators differ from the 
bulk of their allies. 
5. That the imitation, however mi- 
nute, is external and visible only, never 
extending to internal characters or to 
such as do not effect the external 
appearance. 
It is true, Prof. Punnett says, that 
these conditions often hold good, but 
there are few if any cases where they 
all hold good. When the problem 1s 
further examined, still more difficulties 
are found. For instance, the butterfly 
is frequently captured by birds on the 
wing; but though two species may 
resemble each other closely in colora- 
tion their manner of flight is sometimes 
so different that it is hard to believe a 
bird would not see the difference 
between them. 
Breeding experiments offer a further 
objection, Prof. Punnett thinks, to the 
idea that the mimetic pattern has been 
built up by natural selection from a 
long series of small changes. For the 
patterns are found to be inherited as 
Mendelian units and therefore, he 
thinks, must have appeared by one 
large step instead of a number of small 
steps: otherwise we should recover 
some of the intermediate steps by 
cross-breeding. It is not certain, how- 
ever, that this argument deserves as 
much weight as Punnett ascribes to it. 
DO BIRDS DISCRIMINATE?! 
Distinctly more convincing is the 
experimental evidence on the prefer- 
ences of birds. For when they are given 
a chance to select between a mimic and 
a model, they sometimes choose the 
supposedly unpalatable one and reject 
the one which resembles it, but is, by 
hypothesis, comestible. If enough evi- 
dence of this sort could be accumulated, 
it would obviously strike at the very 
foundations of the mimicry hypothesis. 
“It is safe to say,’’ Punnett thinks, 
‘“‘that a number of species of birds have 
been known to attack butterflies—that 
a few out of the number feed upon 
butterflies systematically—that some 
467 
of the most persistent bird enemies 
devour the presumably protected forms 
as freely as the unprotected—but that 
in a few instances there is some reason 
for supposing that the bird discrimi- 
nates. Beyond this it is unsafe to go at 
present.” 
Monkeys eat butterflies readily, and 
appear to be more discriminating than 
birds. It is not impossible that they 
are really responsible for the establish- 
ment of some species of mimicry; for 
Punnett shows by mathematical cal- 
culations that even a small percentage 
handicap of one species is sufficient to 
alter its relative numbers greatly in a 
comparatively small number of genera- 
tions. In 1850, for instance, the pep- 
pered moth Amphidasys betularia was 
common in England; at present it has 
been practically supplanted by a darker 
form, A. doubledayaria. The cause of 
this change is obscure; it is suggested 
that the darker form may be hardier. 
The réle of natural selection in mim1- 
cry is still further limited by a considera- 
tion of what must happen in the early 
stages. Ifa white butterfly is to assume 
the protective coloration of a dark 
form, it appears that a small spot of 
dark color (which, according to the 
selectionists, would be the start of the 
change) would give no real protection. 
Prof. Punnett declares, ‘‘ Till the mimic 
can be mistaken for the model, natural 
selection plays no part. . . . The part 
now often attributed to natural selec- 
tion is to put a polish on the resemblance 
and to keep it up to the mark by weed- 
ing out those which do not reach the 
required standard.” 
From these facts, and others which 
have not been mentioned in this review, 
the author holds ‘‘that there are dif- 
ficulties in the way of accepting the 
mimicry theory as an explanation of the 
remarkable resemblances which are often 
found between butterflies belonging to 
distinct groups. Of these difficulties 
two stand out beyond the rest, viz., 
the difficulty of finding the agent that 
shall exercise the appropriate powers 
of discrimination, and the difficulty of 
fitting in the theoretical process in- 
volving the incessant accumulation of 
minute variations with what is at 
