MIMICRY AND VARIATION
It is clear from the last few chapters that the theory of mimicry in
butterflies with its interpretation of the building up of these likenesses
by means of natural selection in the form of predaceous birds and other
foes is open to destructive criticism from several points of view. The
evidence from mimicry rings makes it almost certain that in some cases the
resemblance must be founded on an initial variation of such magnitude that
the mimic could straightway be confused with the model. Till the mimic can
be mistaken for the model natural selection plays no part. The evidence
from breeding suggests strongly that in certain cases (_e.g._ _Papilio
polytes_) the likeness arose in the form in which we know it to-day. In
such cases there is no reason for supposing that natural selection has had
anything to do with the formation of the finished mimic. Considerations of
this nature may be said to have destroyed the view, current until quite
recently, that in the formation of a mimetic resemblance the exclusive
agent was natural selection. During the past few years it has come to be
admitted by the staunchest upholders of the theory of mimicry that natural
{126} selection would not come into play until the would-be mimic was
sufficiently like the model to be confused with it under natural
conditions[80]. The part now often attributed to natural selection is to
put a polish on the resemblance and to keep it up to the mark by weeding
out those which do not reach the required standard. It is supposed that if
natural selection ceases to operate the mimetic resemblance is gradually
lost owing to the appearance of variations which are no longer weeded out.
An interesting case has recently been brought forward by Carpenter[81] and
explained on these lines: The Nymphaline _Pseudacraea eurytus_ is a
polymorphic species found in Central Africa. In Uganda it occurs in several
distinct forms which were originally supposed to be distinct species. Three
of these forms bear a marked resemblance to three species of the Acraeine
genus _Planema_.
Mimic Model
_Pseudacraea eurytus_ _Planema_
Form _hobleyi_[82] (Pl. VII, figs. 6, 7) _macarista_ (Pl. VII, fig. 2)
_terra_ (Pl. VII, fig. 8) _tellus_ (Pl. VII, fig. 3)
_obscura_ _paragea_ (Pl. VII, fig. 4)
These different species occur round Victoria Nyanza and also on some of the
islands in the lake. Some {127} interesting points are brought out by a
comparison of the occurrence and variation of the species on the mainland
with what is found on Bugalla Island in the Sesse Archipelago. On the
mainland the Pseudacraeas are abundant but the Planemas even more so,
outnumbering the former by about 5:2[83]. Moreover, it is rare to find
individuals more or less intermediate between the three forms, though they
are known to occur. On Bugalla Island, however, a different state of things
is found. The Pseudacraeas are very abundant, whereas the Planemas, owing
doubtless to the scarcity of their food plant, are relatively rare, and are
very greatly outnumbered by the Pseudacraeas. At the same time the
proportion of transitional forms among the Pseudacraeas is definitely
higher than on the mainland. These facts are interpreted by Carpenter as
follows:--
On the mainland where the models are abundant there is a vigorous action on
the part of natural selection. The mimetic forms have a strong advantage
and the non-mimetics have been gradually weeded out. But on the island,
where the Pseudacraeas outnumber the models, the advantage obtained through
mimicry is not so great. The so-called transitional forms are little, if at
all, worse off than those closely resembling the scarce models, and
consequently have as good a chance of surviving as any of the typical
mimetic forms. On {128} the mainland, however, the enemies of _Pseudacraea_
are well acquainted with the _Planema_ models which are here common, and
discriminate against individuals which are not close mimics of the
Planemas. The result is that on the mainland transitional forms are scarcer
than on the island. Natural selection maintains a high standard for the
mimetic likeness on the mainland owing to the abundance of the model; but
when the model is scarce the likeness ceases to be kept up to the mark
strictly, and tends to become lost owing to the appearance of fresh
variations which are no longer weeded out.
Here it should be stated that the various Pseudacraeas form a population in
which the different forms mate freely with one another. In the few breeding
experiments that Dr Carpenter was able to make he found that _obscura_
could produce _terra_, and that _tirikensis_ was able to give _obscura_,
the male in each case being, of course, unknown. Far too little work has as
yet been done on the genetics of these various forms, and it would be rash
to make assumptions as to the nature of the intermediates until the method
of experimental breeding has been more extensively employed in analysing
their constitution. Possibly it is not without significance that the
abundance or scarcity of the _obscura_ form runs parallel with the
abundance or scarcity of the intermediates. It suggests that the
intermediates are heterozygous in some factor for which the typical
_obscura_ is homozygous, and the fact that the intermediates are more
numerous than {129} _obscura_ is what is to be looked for in a population
mating at random. This case of the polymorphic _Pseudacraea eurytus_ is one
of the greatest interest, but it would be hazardous to draw any
far-reaching deductions from such facts as are known at present. When the
genetics of the various typical forms and of the intermediates has been
worked out it will be disappointing if it does not throw clear and
important light on these problems of mimetic resemblance.
As the result of modern experimental breeding work it is recognised that an
intermediate form between two definite varieties may be so because it is
heterozygous for a factor for which one variety is homozygous and which is
lacking in the other--because it has received from only one parent what the
two typical varieties receive from both parents or from neither. Its germ
cells, however, are such as are produced by the two typical forms, and the
intermediate cannot be regarded as a stage in the evolution of one variety
from the other. In these cases of mimicry the existence of intermediate
forms does not entail the deduction that they have played a part in the
evolution of one pattern from another under the influence of a given model.
It is quite possible that the new mimetic pattern appeared suddenly as a
sport and that the intermediates arose when the new form bred with that
which was already in existence. But before we are acquainted with the
genetic relationships between the various forms, both types and
intermediates, speculation as to their origin must remain comparatively
worthless. {130}
In this connection a few words on another source of variation may not be
out of place. The patterns of butterflies are often very sensitive to
changes in the conditions to which they are exposed during later larval and
pupal life. Many moths and butterflies in temperate climates are double
brooded. The eggs laid by the late summer brood hatch out, hibernate in the
larval or pupal state, and emerge in the following spring. This spring
brood produces the summer brood during the same year. In these cases it
often happens that the two broods differ in appearance from one another, a
phenomenon to which the term "Seasonal Dimorphism" has been applied. A
well-marked instance is that of the little European Vanessid, _Araschnia
levana_. The so-called _levana_ form which emerges in the spring is a small
black and orange-brown butterfly (Pl. VI, fig. 10). From the eggs laid by
this brood is produced another brood which emerges later on in the summer,
and is, from its very different appearance, distinguished as the _prorsa_
form (Pl. VI, fig. 9). It is very much darker than the spring form and is
characterised by white bands across the wings. The eggs laid by the
_prorsa_ form give rise to the _levana_ form which emerges in the following
spring. It has been shewn by various workers, and more especially by the
extensive experiments of Merrifield[84], that the appearance of the
_levana_ or the _prorsa_ form from any batch of eggs, whether laid by
_prorsa_ or _levana_, is dependent upon the conditions of temperature under
{131} which the later larval and early pupal stages are passed. By cooling
appropriately at the right stage _levana_ can be made to produce _levana_
instead of the _prorsa_ which it normally produces under summer conditions.
So also by appropriate warming _prorsa_ will give rise to _prorsa_.
Moreover, if the conditions are properly adjusted an intermediate form
_porima_ can be produced, a form which occurs occasionally under natural
conditions. The pattern is, in short, a function of the temperature to
which certain earlier sensitive stages in this species are submitted. What
is true of _A. levana_ is true also of a number of other species. In some
cases temperature is the factor that induces the variation. In other
countries where the year is marked by wet and dry seasons instead of warm
and cold ones moisture is the agent that brings about the change. In some
of the South African butterflies of the genus _Precis_ the seasonal change
may be even more conspicuous than in _A. levana_. In _Precis octavia_, for
example, the ground colour of the wet season form is predominantly red,
while in the dry season form of the same species the pattern is different,
blue being the predominating colour (cf. Pl. VI, figs. 11 and 12). Such
examples as these are sufficient to shew how sensitive many butterflies are
to changes in the conditions of later larval and earlier pupal life. The
variations brought about in this way are as a rule smaller than in the
examples chosen, but in no case are they known to be inherited, and in no
case consequently could variation of this nature play any part in {132}
evolutionary change. Before any given variation can be claimed as a
possible stage in the development of a mimetic likeness satisfactory
evidence must be forthcoming that it is not of this nature, but that it is
transmissible and independent of climatic and other conditions.
Many species of butterflies, especially such as are found over a wide
range, exhibit minor varieties which are characteristic of given
localities. These minor varieties may be quite small. In _Danais
chrysippus_, for example, African and Asiatic specimens can generally be
distinguished. On examples from India a small spot is seen just below the
bar on the fore wing and on the inner side of it. Eastwards towards China
this spot tends to become larger and confluent with the white bar, giving
rise to an L-shaped marking; westwards in Africa the spot tends to
disappear altogether. The existence of such local races has been used as an
argument for the hereditary transmission of very small variations--in the
present instance the size of a small white spot[85]. For if it can be
supposed that small differences of this nature are always transmitted, it
becomes less difficult to imagine that a mimetic resemblance has been
brought about by a long series of very small steps. But before this can be
admitted it is necessary to shew by experiment that the size of this spot
is independent of environmental conditions, both climatic and other. Apart
from temperature and moisture it is not improbable that the formation of
{133} pigment in the wings may depend in some degree upon the nature of the
food. The larvae of _D. chrysippus_ feed upon various Asclepiads, and it is
at any rate conceivable that the pigment formation, and consequently the
details of pattern, may be in slight measure affected by the plant species
upon which they have fed. The species of food plants are more likely to be
different at the extremities of the range of a widely distributed form like
_D. chrysippus_, and if they are really a factor in the pattern it is at
the extremities that we should expect to find the most distinct forms[86].
Actually we do find this in _D. chrysippus_, though it does not, of course,
follow that the cause suggested is the true one, or, if true, the only one.
Of the nature of local races too little at present is known to enable us to
lay down any generalization. We must first learn by experiment how far they
remain constant when transported from their own environment and bred in the
environment under which another distinct local race is living. The
behaviour of the transported race under the altered conditions would help
us in deciding whether any variation by which it is characterised had a
definite hereditary basis or was merely a fluctuation dependent upon
something in the conditions under which it had grown up. The decision as to
whether it is hereditary or not must depend upon the {134} test of
breeding, through which alone we can hope to arrive at a satisfactory
verdict upon any given case.
The particular geographical variation which has just been considered
happens to be a small one. But it may happen that a geographical variety is
much more distinct. Indeed it is not impossible that butterflies which are
at present ranked as distinct species may prove eventually to be different
forms of the same species. Especially is this likely to be true of many
forms in South America, of which Bates long ago remarked "that the
suspicion of many of the species being nothing more than local
modifications of other forms has proved to be well founded." Since Bates'
day more material has been forthcoming[87] and it has been shewn that
certain colour schemes are characteristic of distinct geographical regions
in South America where they may occur in species belonging to very
different genera and families. In Central America, for example, the pattern
common to many species is determined by horizontal and oblique black bands
on a bright fulvous brown ground, with two broken yellow bars towards the
tip of the fore wing. The general type is well shewn by _Mechanitis
saturata_ and the female of _Dismorphia praxinoe_ (Pl. X, figs. 7 and 3).
Belonging to this pattern group are a number of different species belonging
to various families, including several Heliconines and Ithomiines, Pierids
such as _Dismorphia_ and _Perrhybris_, Nymphalines of the genera _Eresia_
and {135} _Protogonius_, and other forms. In Eastern Brazil the predominant
pattern is one characterised by a yellow band across the hind wing and a
white or yellow apical fore wing marking (cf. Pl. XV, figs. 3 and 8). Here
also, with the exception of the _Perrhybris_, all the various genera which
figured in the last group are again represented. It is true that the
members of this second group are regarded as belonging to different species
from those of the first group, but as species here are made by the
systematist chiefly, if not entirely, on the colour pattern this fact may
not mean much. Passing now to Ega on the Upper Amazons the general ground
colour is a deep chestnut purple and the apical area of the fore wings
presents a much mottled appearance (cf. Pl. XV, figs. 4 and 9). In this
group again we find represented the different genera found in the other
groups, the only notable absentees being _Eresia_ and _Perrhybris_. Lastly
in Ecuador, Peru, and Bolivia the general pattern scheme consists of
orange-tawny markings on a black ground (cf. Pl. XV, figs. 5 and 10). This
group differs somewhat in composition from the preceding in that it
contains no Pierid and no Danaid. On the other hand its numbers have been
strengthened by the accession of a _Papilio_, an _Acraea_, and two species
of the Satyrid genus _Pedaliodes_. Certain writers have seen in the theory
of mimicry the only explanation of these peculiar geographical pattern
groups. The fashion is in each case set by the most abundant form,
generally an Ithomiine of the genus _Melinaea_. The rest are mimics of this
dominant species, either in the {136} Batesian or Müllerian sense. Batesian
mimics are such genera as _Dismorphia_ and _Protogonius_, to which there
are no reasons for attributing disagreeable properties. Of the nature of
Müllerian mimics on the other hand are the various Heliconines and
Ithomiines which enter into the combination. In each case the whole
assemblage is a great "mimicry ring," of which the pattern is dictated by
the Ithomiine that predominates in point of numbers. It is, however, very
doubtful whether this can be accepted as a satisfactory explanation. The
four groups which we have considered are all characterised by a peculiar
and distinctive coloration, and in each case the pattern must on the theory
of mimicry be regarded as a highly efficient warning pattern. One or other
of these patterns must doubtless be looked upon as the most primitive. If
so the question at once arises as to why a distasteful genus should change
from one efficient warning pattern to another quite distinct one. If the
newer pattern affords better protection we should expect it to have spread
and eventually to have ousted the older one. That it has not done so must
probably be attributed to the old pattern being as efficient as the new
one. But if this is so we are left without grounds for assuming the change
to have been brought about by natural selection through the agency of
enemies to whom warning colours appeal. For natural selection can only
bring about a change that is beneficial to the species. Hence we must
suppose the change on the part of the dominant model to have been
independent of natural selection by {137} enemies, and due to some
condition or set of conditions of which we are ignorant. It is not
inconceivable that the new colour scheme was associated with some
physiological peculiarity which was advantageous to the species in its
altered surroundings. If so natural selection may have favoured the new
variety, not because of its colour scheme, but owing to the underlying
physiological differences of which the pattern is but an outward sign. And
if this could happen in one species there seems to be no reason why it
should not happen in others. The weak point of the explanation on the
mimicry hypothesis is that it offers no explanation of the change in the
so-called dominant Ithomiine pattern as we pass from one region to another.
Whatever the cause of this change may be there would appear to be nothing
against it having also operated to produce similar changes in other
unrelated species, in which case the mimicry hypothesis becomes
superfluous. It is not unlikely that the establishing of these new forms
was due to natural selection. If they were associated with physiological
peculiarities better adapted for their environment it is reasonable to
suppose that natural selection would favour their persistence as opposed to
the older type until the latter was eliminated. But such action on the part
of natural selection is quite distinct from that postulated on the mimicry
hypothesis. On the one view the colour itself is selected because it is of
direct advantage to its possessor; on the other view the colour pattern is
associated with a certain physiological constitution which places the {138}
butterflies possessing it at an advantage as compared with the rest[88].
It is, nevertheless, possible that mimicry may have played some part in
connection with establishing the new colour pattern in some of these South
American species. For if the new pattern had become established in the
predominant distasteful species, and if some of the members of a palatable
form (_e.g._ _Protogonius_) were to shew a variation similar to that
already established in the distasteful species, and if further there be
granted the existence of appropriate enemies, then it would be almost
certain that the newer form in palatable species would eventually replace
the older form. In such a case the part played by natural selection would
be the preservation of a chance sport which happened to look like an
unpalatable form. There is no reason for regarding the change as
necessarily brought about by the gradual accumulation of a long series of
very small variations through the operation of natural selection.
* * * * * {139}