(8h) [slii, xliii 
minimize the present, or in some cases the local destruction 
of butterflies by birds, and consequently also the number of 
observed cases of such destruction, but it would not preclude 
the possibility that a much more extensive process of selection 
took place amongst butterflies in earlier times. In other 
words the balance of nature is now maintained by other and 
various agencies acting in concert with the now much-reduced 
influence of insectivorous birds, and we are present now only 
to see a state of affairs brought about by agencies which have 
become considerably modified in their relative influences. 
It will probably be immediately pointed out that if the 
selective influence of birds is not as great as ever it was the 
well-known tendency of reversion to ancestral forms would 
tend to do away with mimicry, and that only by constant 
pressure of selective forces can mimetic resemblance be main- 
tained. But is there any evidence that mimicry amongst 
butterflies is at present more extensive and complete than it 
has ever been before? Have we any right to say that all 
cases of imperfect mimicry are in process of being perfected 
and not undergoing reversion? I have never been satisfied 
that the white-winged form of Hypolimnas misippus is really 
a mimic of the alcippus variety of Danais chrysippus, ex- 
tremely attractive as the suggestion undoubtedly is. One 
cannot forget that the male misippus has large white patches, 
and the lack of geographical coincidence between alcippus and 
misippus is much against the mimetic theory. The alcippus 
form may be also a reversion, or it may be, as suggested 
by Professor Poulton, an effort towards more conspicuous 
coloration. If such be the case, we have here an instance of 
accidental resemblance, a phenomenon which I cannot but 
believe does occasionally occur. Such a suggestion will 
probably be looked upon with disfavour by keen supporters 
of Bates’s hypothesis, but I do not see that that hypothesis 
loses anything by the admission that every case of resemblance 
is not necessarily true mimicry. Many other cases of im- 
perfect resemblance might be cited which can just as easily 
be regarded as instances of partial reversion as of incomplete 
development. I do not necessarily insist on the theory that 
birds have partially ceased to be a selective force acting on 
