III. ORIGIN OF SPECIES.

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The theory of the origin of species as propounded by Darwin may be thus very briefly summarised:—

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 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.

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.

                                                                                                                                                                                                                                                                                                           

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