The theory of the origin of species as propounded by Darwin may be thus
very briefly summarised:--
{xiv}VARIATION.--No two organisms are exactly alike; there is always some
variation from the parent form, in some cases very slight, in others
considerable. (For examples of variation see Plate VII., figs. 1 to 9,
varieties of _Hydriomena deltoidata_; Plate VIII., figs. 42 to 47,
varieties of _Epirranthis alectoraria_; Plate IX., figs. 6 to 14, varieties
of _Selidosema productata_; Plate X., figs. 13 to 23, varieties of _Azelina
gallaria_; Plate X., figs. 39 to 47, varieties of _Declana floccosa_.)
INHERITANCE.--Many of these variations are inherited--a fact demonstrated
by our domestic plants and animals, where man has selected and bred from
varieties suitable for his purposes, and has thus produced races in which
the variation is permanent. Many of the races of domestic animals differ as
much from one another as do some distinct species of wild animals.
STRUGGLE FOR EXISTENCE.--All animals and plants produce far more offspring
than can possibly survive, thus giving rise to the struggle for existence.
For example: The average number of eggs laid by a Lepidopterous insect is
certainly over 100, and in many species this number is greatly exceeded.
Assuming each female to lay 100 eggs, the progeny from a single pair would
amount, after six generations, to over six million individuals.
NATURAL SELECTION, or the SURVIVAL OF THE FITTEST.--In the struggle for
existence which necessarily results from such a great increase of
individuals, those variations which favoured the possessors would be
preserved, whilst those which did not, would be gradually exterminated.
This principle of the preservation of the favourable varieties in the
struggle for life is called Natural Selection, or the Survival of the
Fittest.
DIVERGENCE OF CHARACTER.--As there are so many different places and
conditions in the economy of nature which can be occupied by organic beings
differently constituted, individuals which diverged most from the original
type would be brought into less severe competition, than those which
diverged only in a slight degree. For instance, if we represent the
original form as A, occupying one place in the economy of nature; a second
form as B, occupying a somewhat similar place; a third form as C, occupying
a very different place to A although somewhat similar place to B, it is
obvious that B would enter into severe competition with both A and C,
whilst A and C might not trend to any great extent on one another's place
in the natural economy; hence B would be exterminated before either A or C.
In other words, natural selection continually tends to increase the slight
differences, which we call varieties, into the greater differences, which
we call species.
The following phenomena, which have long been observed by students of the
_Lepidoptera_, will serve as excellent examples of the operation of natural
selection:--
PROTECTIVE RESEMBLANCE.--This term is applied to those classes of form or
colour which enable an animal to so closely resemble its surroundings as to
escape the notice of its enemies. Numerous examples of protective
resemblance exist in the New Zealand moths and butterflies; in fact, it may
safely be asserted that nearly all the colouring we observe in these
insects has been acquired for protective purposes. The following species,
amongst many others which will be described hereafter, exhibit in a very
marked degree the phenomenon of protective resemblance: _Epirranthis
alectoraria_, _Selidosema dejectaria_, and _Drepanodes muriferata_ resemble
dead leaves; _Chloroclystis {xv}bilineolata_, _Tatosoma agrionata_, and
_Erana graminosa_ resemble, when at rest, patches of moss; _Selidosema
productata_ and _S. lupinata_ resemble the bark of trees; _Chloroclystis
lichenodes_, _Declana floccosa_, and _Elvia glaucata_ resemble variously
coloured lichens. It is almost unnecessary to point out that all those
variations, which tended to conceal the possessors from their enemies,
would be preserved in the struggle for existence, and that these numerous
and perfect instances of protective resemblance would inevitably result
from the operation of natural selection. The dark colouration of Alpine and
Arctic _Lepidoptera_, which enables them to rapidly absorb heat during the
short and fitful gleams of sunshine experienced on mountains or in high
latitudes, is also an instance of adaptation to conditions through the
influence of natural selection. This was first pointed out by Lord
Walsingham in 1885. The almost complete absence of white species in these
localities is a good example of the extinction of forms unfitted to their
surroundings.
CONTRAST COLOURS.--In this class of colouring the fore-wings only are
protectively coloured, the hind-wings being very conspicuous. Contrast
colouring is well exemplified by several of the insects included in the
genus _Notoreas_. The sudden exhibition of the hind-wings during flight
dazzles the eye of the pursuer. When the insect immediately afterwards
closes its wings and the fore-wings alone are visible, it is extremely
difficult to see. This form of protective colouring was also first drawn
attention to by Lord Walsingham. (See page 75.)
WARNING COLOURS.--Insects, which are unfit for food or nauseous, are not
protectively coloured, but on the contrary are rendered as conspicuous as
possible. This class of colouring is well illustrated by one of our
commonest moths, _Nyctemera annulata_ (Pl. IV., figs. 1 and 2). The
principle of warning colours was first discovered by Mr. A. R. Wallace, and
is graphically described in Professor Poulton's entertaining work, 'The
Colours of Animals.' The possession of nauseous qualities would be of
little value to an insect, unless it could be at once recognised by
insectivorous animals and avoided as food. If a nauseous insect were not
easily identified it would speedily be destroyed by what Professor Poulton
ingeniously terms "experimental tasting"; hence, through the process of
natural selection, all nauseous species have become very conspicuously
coloured. It may be remarked that warning colours are extremely rare
amongst the New Zealand species, and I am not aware of any other example
than that already given.
MIMICRY.--This term is applied to those remarkable cases where a harmless
or edible species imitates in form and colouring a highly armed or nauseous
species. No instances of this extremely interesting class of protection are
yet known amongst the New Zealand _Lepidoptera_, but a very perfect example
of mimicry exists between two common introduced species of _Hymenoptera_
and _Diptera_, the well-known honey-bee and the drone-fly. The superficial
resemblance between these two insects is very close. The bee, as every one
knows, is armed with a powerful sting, whilst the drone-fly is unarmed. In
this case it can be seen that if a harmless insect varied in the direction
of resembling a formidable or objectionable species it would be a decided
advantage to it, and such varieties would tend to be continually preserved
and improved, through the operation of natural selection. The subject of
mimicry has been alluded to here as it is not impossible that some
instances of it may yet be discovered in connection with our native
_Lepidoptera_.
{xvi}ORNAMENTAL COLOURING.--This class of colouring occurs in many species,
especially amongst the butterflies, and is not apparently connected in any
way with protection. Darwin supposes that it has arisen through the females
of each species always selecting the most beautiful males as mates, hence
these alone would leave progeny, and the females themselves would
afterwards become beautiful through the effects of inheritance. This
principle Darwin has termed Sexual Selection, and has discussed it in great
detail in his work on the 'Descent of Man.' The fact, that amongst birds
and butterflies the males are nearly always the most brilliantly coloured
and the most beautiful, together with an immense mass of other evidence,
tends, I think, to entirely support Darwin's theory, although it should be
mentioned that several eminent naturalists, including Mr. Wallace, do not
admit the principle of Sexual Selection.