Chapter V
Reading edition
SOME CRITICISMS
(1) That the imitative species occur in the same area and occupy the very same station as the imitated.
(2) That the imitators are always the more defenceless.
(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 minute, is external and visible only, never extending to internal characters or to such as do not affect the external appearance.
In offering certain criticisms of the mimicry explanation it will be convenient to do so in connection with these five conditions which Wallace regarded as constant for all cases of mimetic resemblance.
(1) That the imitative species occur in the same area and occupy the very same station as the imitated.
Though in the great majority of cases the imitator and the imitated occur in the same locality, this is not always so. The female of the Fritillary Argynnis hyperbius (Pl. IV, fig. 3), for instance, is exceedingly difficult to distinguish from Danais plexippus when flying, although when at rest the difference between the two is sufficiently obvious. Both insects are plentiful in Ceylon but inhabit different stations. The Danaid is a low-country insect, while the Fritillary is not found until several thousand feet up. The two species affect entirely different stations and hardly come into contact with each other. Where one is plentiful the other is not found. It has been suggested that migratory birds may have come into play in such cases. The bird learns in the low country that D. plexippus is unpleasant, and when it pays a visit to the hills it takes this experience with it and avoids those females of the Fritillary which recall the unpleasant Danaine.
(2) That the imitators are always the more defenceless.
(3) That the imitators are always less numerous in individuals.
It has sometimes been suggested in explanation of the greater abundance of the mimic that in such cases we are concerned with Müllerian mimicry, that since both of the species concerned are distasteful there is not, strictly speaking, either a mimic or a model, and consequently the relative proportions have not the significance that they possess where the mimicry is of the simple Batesian type. It is, however, very doubtful whether such an explanation is of any value, for, as will appear later, there are grave objections to accepting the current theory as to the way in which a resemblance is established on Müllerian lines (cf. pp. 72-74).
(4) That the imitators differ from the bulk of their allies.
The fifth of Wallace's conditions is clear and needs no discussion.
It is evident that at any rate a large proportion of the instances of close resemblance do not fulfil all of the conditions laid down by Wallace. Nevertheless we should expect them to do so if the resemblance has been brought about by the cumulative effect of natural selection on small favourable variations. Clearly there is a prima facie case for doubting whether we must of necessity ascribe all resemblance of the kind to natural selection, and in the next few chapters we shall discuss it in more detail from several points of view.