"MIMICRY RINGS"
Having reviewed briefly some of the most striking phenomena of what has
been termed mimicry, we may now inquire whether there are good grounds for
supposing that these resemblances have been brought about through the
operation of natural selection or whether they are due to some other cause.
If we propose to offer an explanation in terms of natural selection we are
thereby committed to the view that these resemblances are of the nature of
adaptation. For unless we grant this we cannot suppose that natural
selection has had anything to do either with their origin or with their
survival. Granting then for the present the adaptational nature of these
mimetic resemblances, we may attempt to deduce from them what we can as to
the mode of operation of natural selection. In doing so we shall bear in
mind what may be called the two extreme views: viz. (_a_) that the
resemblance has been brought about through the gradual accumulation of very
numerous small variations in the right direction through the operation of
natural selection, and (_b_) that the mimetic form came into being as a
sudden sport or {62} mutation, and that natural selection is responsible
merely for its survival and the elimination of the less favoured form from
which it sprang.
There is a serious difficulty in the way of accepting the former of these
two views. If our two species, model and would-be mimic are, to begin with,
markedly different in pattern, how can we suppose that a slight variation
in the direction of the model on the part of the latter would be of any
value to it? Take for example a well-known South American case--the
resemblance between the yellow, black, and brown Ithomiine, _Mechanitis
saturata_ (Pl. X, fig. 7) and the Pierine, _Dismorphia praxinoe_ (Pl. X,
fig. 3). The latter belongs to the family of the "whites," and
entomologists consider that in all probability its ancestral garb was white
with a little black like the closely allied species _D. cretacea_ (Pl. X,
fig. 1). Can we suppose that in such a case a small development of brown
and black on the wings would be sufficient to recall the Ithomiine and so
be of service to the _Dismorphia_ which possessed it? Such a relatively
slight approach to the Ithomiine colouring is shewn by the males of certain
South American "whites" belonging to the genus _Perrhybris_ (Pl. X, figs. 4
and 5). But the colour is confined to the under-surface and the butterflies
possessing it could hardly be confused with a _Mechanitis_ more than their
white relations which entirely lack such a patch of colour. If birds
regarded white butterflies as edible it is difficult to suppose that they
would be checked in their attacks {63} by a trifling patch of colour while
the main ground of the insect was still white. But unless they avoided
those with the small colour patch there would be an end of natural
selection in so far as the patch was concerned, and it would have no
opportunity of developing further through the operation of that factor.
This is the difficulty of the initial variation which has been clearly
recognised by most of the best known supporters of the theory of mimicry.
Bates himself offered no suggestion as to the way in which such a form as a
Pierid could be conceived of as beginning to resemble an Ithomiine[30].
Wallace supposed that the Ithomiines were to start with not so distinct
from many of the edible forms as they are to-day, and that some of the
Pierines inhabiting the same district happened to be sufficiently like some
of the unpalatable forms to be mistaken for them occasionally[31].
The difficulty of the initial variation had also occurred to Darwin, and he
discusses it in an interesting passage which is so important that we may
quote it here in full:
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But starting with
species already somewhat like each other, the closest resemblance, if
beneficial, could readily be gained by the above means; and if the
imitated form was subsequently and gradually {64} modified through any
agency, the imitating form would be led along the same track, and thus
be altered to almost any extent, so that it might ultimately assume an
appearance or colouring wholly unlike that of the other members of the
family to which it belonged. There is, however, some difficulty on this
head, for it is necessary to suppose in some cases that ancient members
belonging to several distinct groups, before they had diverged to their
present extent, accidentally resembled a member of another and
protected group in a sufficient degree to afford some slight
protection; this having given the basis for the subsequent acquisition
of the most perfect resemblance[32].
Both Darwin and Wallace recognised clearly this difficulty of the initial
variations, and both suggested a means of getting over it on similar lines.
Both supposed that in general colour and pattern the groups to which model
and mimic belonged were far more alike originally than they are to-day.
They were in fact so much alike that comparatively small variations in a
favourable direction on the part of the mimic would lead to its being
confused with the unpalatable model. Then as the model became more and more
conspicuously coloured, as it developed a more and more striking pattern
warning would-be enemies of its unpleasant taste, the mimic gradually kept
pace with it through the operation of natural selection, in the shape of
the discriminating enemy, eliminating those most unlike the model. The
mimic travelled closely in the wake of the model, coaxed as it were by
natural selection, till at last it was far removed in general appearance
from the great majority of its near relations. {65}
In this way was offered a comparatively simple method of getting over the
difficulty of applying the principle of natural selection to the initial
variations in a mimetic approach on the part of one species to another. But
it did not escape Darwin's penetration that such an argument would not
always be easy of application--that there might be cases where a given
model was mimicked by members of several groups of widely differing
ancestral pattern, and that in these cases it would be difficult to
conceive of members of each of the several groups shewing simultaneous
variations which would render them liable to be mistaken for the protected
model. The difficulty may perhaps be best illustrated if we consider a
definite case.
It is a feature of mimetic resemblances among butterflies that a given
species in a given locality may serve as a model for several other species
belonging to unrelated groups. Generally such mimics belong to presumably
palatable species, but other presumably unpalatable species may also
exhibit a similar coloration and pattern. In this way is formed a combine
to which the term "mimicry ring" has sometimes been applied. An excellent
example of such a mimicry ring is afforded by certain species of
butterflies in Ceylon, and is illustrated on Plate IV. It is made up in the
first place of two species belonging to the presumably distasteful Danaine
group, viz. _Danais chrysippus_ and _D. plexippus_. The latter is a rather
darker insect but presents an unmistakable general likeness to _D.
chrysippus_. Those who believe in {66} Müllerian mimicry would regard it as
an excellent example of that phenomenon. For those who believe only in
Batesian mimicry _D. plexippus_, being the scarcer insect, must be regarded
as the mimic and _D. chrysippus_ as the model. In both of these species the
sexes are similar, whereas in the other three members of the "ring" the
female alone exhibits the resemblance. One of these three species is the
common Nymphaline, _Hypolimnas misippus_, of which the female bears an
extraordinary likeness to _D. chrysippus_ when set and pinned out on cork
in the ordinary way. The male, however (Pl. IV, fig. 8), is an insect of
totally different appearance. The upper surfaces of the wings are velvety
black with a large white patch bordered with purple in the middle of
each[33]. The "ring" is completed by the females of _Elymnias undularis_
and _Argynnis hyperbius_. The former of these belongs to the group of
Satyrine butterflies and the female is usually regarded as a mimic of _D.
plexippus_, which it is not unlike in so far as the upper surface of the
wings is concerned. Here again the male is an insect of totally dissimilar
appearance. Except for a border of lighter brown along the outer edges of
the hind wings the upper surface is of a uniform deep purple-brown {67} all
over (Pl. IV, fig. 6). In _Argynnis hyperbius_ the appearance is in general
that of the Fritillary group to which it belongs. But in the female the
outer portion of the fore wings exhibits much black pigment and is crossed
by a broad white band similar to that found in the same position on the
wing of _D. plexippus_ (Pl. IV, fig. 2).
Of the five species constituting this little "mimicry ring" in Ceylon two,
on the current theory of mimicry, are to be regarded as definitely
unpalatable, one (_H. misippus_) as doubtfully so, while the Satyrine and
the Fritillary are evidently examples of simple or Batesian mimicry.
Now such examples as this of simultaneous mimicry in several species are of
peculiar interest for us when we come to inquire more closely into the
process by which the resemblances can be supposed to have been brought
about. Take for example the case of _E. undularis_. The male is evidently
an unprotected insect in so far as mimicry is concerned, while the female
exhibits the general pattern and coloration characteristic of the warningly
coloured and presumably distasteful species _D. plexippus_ or _D.
chrysippus_. If we are to suppose this to have been brought about by the
operation of natural selection it is clear that we must regard the colour
and pattern of the male as the original colour and pattern of both sexes.
For natural selection cannot be supposed to have operated in causing the
male to pass from a protected to an unprotected condition, or even in
causing him {68} to change one unprotected condition for another. Probably
all adherents of the mimicry theory would be agreed in regarding the male
of _Elymnias undularis_ as shewing the ancestral coloration of the species,
and in looking upon the female as having been modified to her own advantage
in the direction of _D. plexippus_. The question that we have to try to
decide is how this has come about--whether by the accumulation of slight
variations, or whether by a sudden change or mutation in the pattern and
colour of the female by which she came to resemble closely the Danaine. It
is clear that if _D. plexippus_ were what it is to-day before the mimetic
approach on the part of _E. undularis_ began, small variations in the
latter would have been of no service to it. The difference between the two
species would have been far too great for individuals exhibiting slight
variation in the direction of _D. plexippus_ to stand any chance of being
confused with this species. And unless such confusion were possible natural
selection could not work. There is, however, an immediate way out of the
difficulty. We may suppose that the coloration of the male of the mimic,
_E. undularis_, is not only the ancestral colour of its own species but
also of the model. _D. plexippus_ on this supposition was very like _E.
undularis_, of which both sexes were then similar to what the male is
to-day. The pattern is, however, an inconspicuous one, and it can be
imagined that it might be to the advantage of _D. plexippus_ to don a
brighter garb for the advertisement of its unpleasant qualities. {69}
Variations in the direction of a more conspicuous pattern would for that
reason tend to be preserved by natural selection, until eventually was
evolved through its means the well-marked pattern so characteristic of the
model to-day. If in the meantime variations in the same direction occurred
among the females of _E. undularis_ these would tend to be preserved
through their resemblance to the developing warning pattern of the
distasteful Danaine model. The development of model and mimic would proceed
_pari passu_, but if the sexes of the mimic differ, as in this case, we
must suppose the starting-point to have been the condition exhibited by the
male of the mimicking species.
But _Argynnis hyperbius_ is also a species in which the female mimics _D.
plexippus_; and by using the same argument as that just detailed for
_Elymnias undularis_ we can shew that the Danaine model, _D. plexippus_,
must also have been like the male of _Argynnis hyperbius_. And if the
resemblance of _A. hyperbius_ was developed subsequently to that of _E.
undularis_, then both _D. plexippus_ and _E. undularis_ must at one time
have been like the male of _A. hyperbius_, a proposition to which few
entomologists are likely to assent. Further, since the female of _H.
misippus_ also comes into the _plexippus-chrysippus_ combine we must
suppose that these species must at some time or another have passed through
a pattern stage like that of the _misippus_ male.
It is scarcely necessary to pursue this argument {70} further, for even the
most devoted adherents of the theory of mimicry as brought about by the
operation of natural selection on small variations are hardly likely to
subscribe to the phylogenetic consequences which it must entail in cases
where a model is mimicked by the females of several species whose males are
widely dissimilar in appearance.
Even if we suppose the two Danaines to have been originally like the male
of one of the three mimics, we must still suppose that the females of the
other two originated as "sports," sufficiently near to Danaines to be
confused with them. But if such sports can be produced suddenly by some
mutational process not at present understood, why should not these sports
be the females of the three mimicking species as we see them at present?
Why need we suppose that there were intermediate stages between the
mimicking female and the original hypothetical female which was like the
male? If a sport occurred which was sufficiently similar to an unpalatable
species to be confused with it, it is theoretically demonstrable that,
although relatively scarce to start with, it would rapidly increase at the
expense of the unprotected male-like female until the latter was
eliminated. We shall, however, return in a later chapter (p. 96) to the
argument by which this view can be supported.
So far we have discussed what we called the two extreme views as to the way
in which a mimetic resemblance may be supposed to have originated. Of the
two that which assumes the resemblance to have {71} been brought about by a
succession of slight variations must also assume that model and mimic were
closely alike to start with, and this certainly cannot be true in many
cases. On the other hand, there is so far no reason against the idea of
supposing the resemblance to have originated suddenly except what to most
minds will probably appear its inherent improbability.
There are writers on these questions of mimicry who adopt a view more or
less intermediate between those just discussed. They regard the resemblance
as having arisen in the first place as a sport of some magnitude on the
part of the mimic, rendering it sufficiently like the model to cause some
confusion between the two. A rough-hewn resemblance is first brought about
by a process of mutation. Natural selection is in this way given something
to work on, and forthwith proceeds to polish up the resemblance until it
becomes exceedingly close. Natural selection does not originate the
likeness, but, as soon as a rough one has made its appearance, it comes
into operation and works it up through intermediate stages into the
finished portrait. It still plays some part in the formation of a mimetic
resemblance though its rôle is now restricted to the putting on of the
finishing touches. Those who take this view hold also that the continued
action of natural selection is necessary in order to keep the likeness up
to the mark. They suppose that if selection ceases the likeness gradually
deteriorates owing to the coming into operation of a mysterious {72}
process called regression. This idea involves certain conceptions as to the
nature of variation which we shall discuss later.
Though it is difficult to regard Batesian mimicry as produced by the
accumulation of small variations through natural selection, it is perhaps
rather more plausible to suppose that such a process may happen in
connection with the numerous instances of Müllerian mimicry. For since the
end result is theoretically to the advantage of both species instead of but
one, it is possible to argue that the process would be simplified by their
meeting one another halfway, as Müller[34] himself originally suggested.
Variations on the part of each in the direction of the other would be
favourably selected, the mimicry being reciprocal.
Difficulties, however, begin to arise when we inquire into the way in which
this unification of pattern may be conceived of as having come about. By no
one have these difficulties been more forcibly presented than by
Marshall[35] in an able paper published a few years ago, and perhaps the
best way of appreciating them is to take a hypothetical case used by him as
an illustration.
Let us suppose that in the same area live two equally distasteful species A
and _B_, each with a conspicuous though distinct warning pattern, and each
sacrificing 1000 individuals yearly to the education of young {73} birds.
Further let it be supposed that A is a common species of which there are
100,000 individuals in the given area, while _B_ is much rarer, and is
represented by 5000. The toll exacted by young birds falls relatively more
lightly upon A than upon _B_, for A loses only 1%, whereas _B_'s loss is
20%. Clearly if some members of _B_ varied so that they could be mistaken
for A it would be greatly to their advantage, since they would pass from a
population in which the destruction by young birds was 20% to one in which
it would now be rather less than 1%. Moreover, as the proportion of _B_
resembling A gradually increased owing to this advantage, the losses
suffered by those exhibiting the original _B_ pattern would be relatively
heavier and heavier until the form was ultimately eliminated. In other
words, it is theoretically conceivable that of two distasteful species with
different patterns the rarer could be brought to resemble the more
abundant.
We may consider now what would happen in the converse case in which the
more numerous species exhibited a variation owing to which it was confused
with the rarer. Suppose that of the 100,000 individuals of A 10,000 shewed
a variation which led to their being mistaken for _B_, so that there are
90,000 of the A pattern and 15,000 of the _B_ pattern of which 10,000
belong to species A. A will now lose 1000 out of the 90,000 having the A
pattern, and 2/3 × 1000 out of the 10,000 of species A which exhibit the
_B_ pattern. The toll of the birds will be 1/90 of those keeping the
original A pattern, and 2/30 of those of species A which have {74} assumed
the _B_ pattern. The mortality among the mimetic members of A is six times
as great as among those which retain the type form. It is clear therefore
that a variation of A which can be mistaken for _B_ is at a great
disadvantage as compared with the type form[36], and consequently it must
be supposed that the Müllerian factor, as the destruction due to
experimental tasting by young birds is termed, cannot bring about a
resemblance on the part of a more numerous to a less numerous species.
Further, as Marshall goes on to shew, there can be no approach of one
species to the other when the numbers are approximately equal. A condition
essential for the establishing of a mimetic resemblance on Müllerian lines,
no less than on Batesian, is that the less numerous species should take on
the pattern of the more numerous. Consequently the argument brought forward
in the earlier part of this chapter against the establishing of such a
likeness by a long series of slight variations is equally valid for
Müllerian mimicry[37].
* * * * * {75}