Part 20

Any help to a comprehension of the causes which may have concurred in bringing about so much diversity and beauty must be of value; and this is my excuse for laying before you the more important cases I have met with of a connection between colour and locality.

_The influence of Locality on Colour in Butterflies and Birds._--Our first example is from tropical Africa, where we find two unrelated groups of butterflies belonging to two very distinct families (Nymphalidæ and Papilionidæ) characterized by a prevailing blue-green colour not found in any other continent.[29] Again, we have a group of African Pieridæ which are white or pale yellow with a marginal row of bead-like black spots; and in the same country one of the Lycænidæ (_Leptena erastus_) is coloured so exactly like these that it was at first described as a species of _Pieris_. None of these four groups are known to be in any way specially protected, so that the resemblance cannot be due to protective mimicry.

[29] Romaleosoma and Euryphene (Nymphalidæ), _Papilio zalmoxis_ and several species of the Nireus-group (Papilionidæ).

In South America we have far more striking cases; for in the three subfamilies Danainæ, Acræinæ, and Heliconiinæ, all of which are specially protected, we find identical tints and patterns reproduced, often in the greatest detail, each peculiar type of coloration being characteristic of separate geographical subdivisions of the continent. Nine very distinct genera are implicated in these parallel changes--Lycorea, Ceratinia, Mechanitis, Ithomia, Melinæa, Tithorea, Acræa, Heliconius, and Eueides, groups of three or four (or even five) of them appearing together in the same livery in one district, while in an adjoining district most or all of them undergo a simultaneous change of coloration or of marking. Thus in the genera Ithomia, Mechanitis, and Heliconius we have species with yellow apical spots in Guiana, all represented by allied species with white apical spots in South Brazil. In Mechanitis, Melinæa, and Heliconius, and sometimes in Tithorea, the species of the Southern Andes (Bolivia and Peru) are characterized by an orange and black livery, while those of the Northern Andes (New Granada) are almost always orange-yellow and black. Other changes of a like nature, which it would be tedious to enumerate but which are very striking when specimens are examined, occur in species of the same groups inhabiting these same localities, as well as Central America and the Antilles. The resemblance thus produced between widely different insects is sometimes general, but often so close and minute that only a critical examination of structure can detect the difference between them. Yet this can hardly be true mimicry, because all are alike protected by the nauseous secretion which renders them unpalatable to birds.

In another series of genera (Catagramma, Callithea, and Agrias) all belonging to the Nymphalidæ, we have the most vivid blue ground, with broad bands of orange, crimson or a different tint of blue or purple, exactly reproduced in corresponding, yet unrelated species, occurring in the same locality; yet, as none of these groups are known to be specially protected, this can hardly be true mimicry. A few species of two other genera in the same country (Eunica and Siderone) also reproduce the same colours, but with only a general resemblance in the markings. Yet again, in tropical America we have species of Apatura which, sometimes in both sexes, sometimes in the female only, exactly imitate the peculiar markings of another genus (Heterochroa) confined to America: here, again, neither genus is protected, and the similarity must be due to unknown local causes.

But it is among islands that we find some of the most striking examples of the influence of locality on colour, generally in the direction of paler, but sometimes of darker and more brilliant hues, and often accompanied by an unusual increase of size. Thus in the Moluccas and New Guinea we have several Papilios (_P. euchenor_, _P. ormenus_, and _P. tydeus_) distinguished from their allies by a much paler colour, especially in the females which are almost white. Many species of Danais (forming the subgenus Ideopsis) are also very pale. But the most curious are the Euplœas, which in the larger islands are usually of rich dark colours, while in the small islands of Banda, Ké, and Matabello at least three species not nearly related to each other (_E. hoppferi_, _E. euripon_, and _E. assimilata_) are all broadly banded or suffused with white, their allies in the larger islands being all very much darker. Again, in the genus Diadema, belonging to a distinct family, three species from the small Aru and Ké islands (_D. deois_, _D. hewitsonii_, and _D. polymena_) are all more conspicuously white-marked than their representatives in the larger islands. In the beautiful genus Cethosia, a species from the small island of Waigiou (_C. cyrene_) is the whitest of the genus. Prothoë is represented by a blue species in the continental island of Java, while those inhabiting the ancient insular groups of the Moluccas and New Guinea are all pale yellow or white. The genus Drusilla, almost confined to these islands, comprises many species which are all very pale; while in the small island of Waigiou is found a very distinct genus, Hyantis, which, though differing completely in the neuration of the wings, has exactly the same pale colours and large ocellated spots as Drusilla.

Equally remarkable is the increase of size in some islands. The small island of Amboina produces larger butterflies than any of the much larger islands which surround it. This is the case with at least a dozen butterflies belonging to many distinct genera,[30] so that it is impossible to attribute the fact to other than some local influence. In Celebes, as I have elsewhere pointed out,[31] we have a peculiar form of wing and much larger size running through a whole series of distinct butterflies; and this seems to take the place of any speciality in colour.

[30] _Ornithoptera priamus_, _O. helena_, _Papilio deiphobus_, _P. ulysses_, _P. gambrisius_, _P. codrus_, _Iphias leucippe_, _Euplœa prothoë_, _Hestia idea_, _Athyma jocaste_, _Diadema pandarus_, _Nymphalis pyrrhus_, _N. euryalus_, _Drusilla jairus_.

[31] “Contributions to the Theory of Natural Selection,” pp. 168-173.

In a very small collection of insects recently brought from Duke-of-York Island (situated between New Britain and New Ireland) are several of remarkably white or pale coloration. A species of Euplœa is the whitest of all known species of that extensive genus; while a beautiful diurnal moth is much whiter than its ally in the larger island of New Guinea. There is also a magnificent longicorn beetle almost entirely of an ashy white colour.[32]

[32] These insects are described and figured in the “Proceedings of the Zoological Society,” for 1877, p. 139. Their names are _Euplœa browni_, _Alcides aurora_, and _Batocera browni_.

From the Fiji Islands we have comparatively few butterflies; but there are several species of Diadema of unusually pale colours, some almost white.

The Philippine Islands seem to have the peculiarity of developing metallic colours. We find there at least three species of Euplœa[33] not closely related, and all of more intense metallic lustre than their allies in other islands. Here also we have one of the large yellow Ornithopteræ (_O. magellanus_), whose hind wings glow with an intense opaline lustre not found in any other species of the entire group; and an Adolias[34] is larger and of more brilliant metallic colouring than any other species in the archipelago. In these islands also we find the extensive and wonderful genus of weevils (Pachyrhynchus), which in their brilliant metallic colouring surpass anything found in the whole eastern hemisphere, if not in the whole world.

[33] _Euplœa hewitsonii_, _E. diocletiana_, _E. lætifica_.

[34] _Adolias calliphorus._

In the Andaman Islands in the Bay of Bengal there are a considerable number of peculiar species of butterflies differing slightly from those on the continent, and generally in the direction of paler or more conspicuous colouring. Thus two species of Papilio which on the continent have the tails black, in their Andaman representatives have them either red or white-tipped.[35] Another species[36] is richly blue-banded where its allies are black; while three species of distinct genera of Nymphalidæ[37] all differ from their allies on the continent in being of excessively pale colours as well as of somewhat larger size.

[35] _Papilio rhodifer_ (near _P. doubledayi_), and _Papilio charicles_ (near _P. memnon_).

[36] _Papilio mayo._

[37] _Euplœa andamanensis_, _Cethosia biblis_, _Cyrestis cocles_.

In Madagascar we have the very large and singularly white-spotted _Papilio antenor_; while species of three other genera[38] are very white or conspicuous as compared with their continental allies.

[38] _Danais nossima_, _Melanitis massoura_, _Diadema dexithea_.

Passing to the West-Indian Islands and Central America (which latter country has formed a group of islands in very recent times) we have similar indications. One of the largest of the Papilios inhabits Jamaica,[39] while another, the largest of its group, is found in Mexico.[40] Cuba has two of the same genus whose colours are of surpassing brilliancy;[41] while the fine genus Clothilda--confined to the Antilles and Central America--is remarkable for its rich and showy colouring.

[39] _Papilio homerus._

[40] _P. daunus._

[41] _P. gundlachianus_, _P. villiersi_.

Persons who are not acquainted with the important structural differences that distinguish these various genera of butterflies can hardly realize the importance and the significance of such facts as I have now detailed. It may be well, therefore, to illustrate them by supposing parallel cases to occur among the Mammalia. We might have, for example, in Africa, the gnus, the elands, and the buffaloes, all coloured and marked like zebras, stripe for stripe over the whole body exactly corresponding. So the hares, marmots, and squirrels of Europe might be all red with black feet, while the corresponding species of Central Asia were all yellow with black heads. In North America we might have raccoons, squirrels, and opossums, in particoloured livery of white and black, so as exactly to resemble the skunk of the same country; while in South America they might be black with a yellow throat-patch, so as to resemble with equal closeness the tayra of the Brazilian forests. Were such resemblances to occur in anything like the number and with the wonderful accuracy of imitation met with among the Lepidoptera, they would certainly attract universal attention among naturalists, and would lead to the exhaustive study of the influence of local causes in producing such startling results.

One somewhat similar case does indeed occur among the Mammalia, two singular African animals, the Aard-wolf (Proteles) and the hyæna-dog (Lycaon), both strikingly resembling hyænas in their general form as well as in their spotted markings. Belonging as they all do to the Carnivora, though to three distinct families, it seems quite an analogous case to those we have imagined; but as the Aard-wolf and the hyæna-dog are both weak animals compared with the hyæna, the resemblance may be useful, and in that case would come under the head of mimicry. This seems the more probable because, as a rule, the colours of the Mammalia are protective, and are too little varied to allow of the influence of local causes producing any well-marked effects.

When we come to birds, however, the case is different; for although they do not exhibit such distinct marks of the influence of locality as do butterflies--probably because the causes which determine colour are in their case more complex--yet there are distinct indications of some effect of the kind, and we must devote some little time to their consideration.

One of the most curious cases is that of the parrots of the West-Indian Islands and Central America, several of which have white heads or foreheads, occurring in two distinct genera,[42] while none of the more numerous parrots of South America are so coloured. In the small island of Dominica we have a very large and richly-coloured parrot (_Chrysotis augusta_) corresponding to the large and richly-coloured butterfly (_Papilio homerus_) of Jamaica.

[42] _Pionus albifrons_ and _Chrysotis senilis_ (C. America), _Chrysotis sallæi_ (Hayti).

The Andaman Islands are equally remarkable, at least six of the peculiar birds differing from their continental allies in being much lighter, and sometimes with a large quantity of pure white in the plumage,[43] exactly corresponding to what occurs among the butterflies.

[43] _Kittacincla albiventris_, _Geocichla albigularis_, _Sturnia andamanensis_, _Hyloterpe grisola_ var., _Ianthœnas palumboides_, _Osmotreron chloroptera_.

In the Philippines this is not so marked a feature; yet we have here the only known white-breasted king-crow (_Dicrurus mirabilis_); the newly discovered _Eurylæmus steerii_, wholly white beneath; three species of Diceum, all white beneath; several species of Parus, largely white-spotted; while many of the pigeons have light ashy tints. The birds generally, however, have rich dark colours, similar to those which prevail among the butterflies.

In Celebes we have a swallow-shrike and a peculiar small crow allied to the jackdaw,[44] whiter than any of their allies in the surrounding islands; but otherwise the colours of the birds call for no special remark.

[44] _Artamus monachus_, _Corvus advena_.

In Timor and Flores we have white-headed pigeons,[45] and a long-tailed flycatcher almost entirely white.[46]

[45] _Ptilopus cinctus_, _P. albocinctus_.

[46] _Tchitrea affinis_, var.

In Duke-of-York Island east of New Guinea we find that the four new species figured in the “Proceedings of the Zoological Society,” for 1877, are _all_ remarkable for the unusual quantity of white in their plumage. They consist of a flycatcher, a diceum, a wood-swallow, and a ground pigeon;[47] all equalling if not surpassing their nearest allies in whiteness, although some of these, from the Philippines, Moluccas and Celebes, are sufficiently remarkable in this respect.

[47] _Monarcha verticalis_, _Diceum eximium_, _Artamus insignis_, _Phlogœnas johannæ_.

In the small Lord Howe’s Island we have the recently extinct white rail (_Notornis alba_), remarkably contrasting with its allies in the larger islands of New Zealand.

We cannot, however, lay any stress on isolated examples of white colour, since these occur in most of the great continents; but where we find a series of species of distinct genera all differing from their continental allies in a whiter coloration, as in the Andaman Islands, Duke-of-York Island, and the West Indies, and, among butterflies, in the smaller Moluccas, the Andamans, and Madagascar, we cannot avoid the conclusion that in these insular localities some general cause is at work.

There are other cases, however, in which local influences seem to favour the production or preservation of intense crimson or a very dark coloration. Thus in the Moluccas and New Guinea alone we have bright red parrots belonging to two distinct families,[48] and which therefore most probably have been independently produced or preserved by some common cause. Here, too, and in Australia we have black parrots and pigeons;[49] and it is a most curious and suggestive fact that in another insular subregion--that of Madagascar and the Mascarene Islands--these same colours reappear in the same two groups.[50]

[48] _Lorius_, _Eos_ (Trichoglossidæ), _Eclectus_ (Palæornithidæ).

[49] _Microglossus_, _Calyptorhynchus_, _Turacœna_.

[50] _Coracopsis_, _Alectrœnas_.

_Sense-perception influenced by Colour of the Integuments._--Some very curious physiological facts bearing upon the presence or absence of white colours in the higher animals have lately been adduced by Dr. Ogle.[51] It has been found that a coloured or dark pigment in the olfactory region of the nostrils is essential to perfect smell, and this pigment is rarely deficient except when the whole animal is pure white. In these cases the creature is almost without smell or taste. This, Dr. Ogle believes, explains the curious case of the pigs in Virginia adduced by Mr. Darwin, white pigs being killed by a poisonous root which does not affect black pigs. Mr. Darwin imputed this to a constitutional difference accompanying the dark colour, which rendered what was poisonous to the white-coloured animals quite innocuous to the black. Dr. Ogle, however, observes that there is no proof that the black pigs eat the root, and he believes the more probable explanation to be that it is distasteful to them; while the white pigs, being deficient in smell and taste, eat it and are killed. Analogous facts occur in several distinct families. White sheep are killed in the Tarentino by eating _Hypericum crispum_, while black sheep escape; white rhinoceroses are said to perish from eating _Euphorbia candelabrum_; and white horses are said to suffer from poisonous food where coloured ones escape. Now it is very improbable that a constitutional immunity from poisoning by so many distinct plants should, in the case of such widely different animals, be always correlated with the same difference of colour; but the facts are readily understood if the senses of smell and taste are dependent on the presence of a pigment which is deficient in wholly white animals. The explanation has, however, been carried a step further, by experiments showing that the absorption of odours by dead matter, such as clothing, is greatly affected by colour; black being the most powerful absorbent; then blue, red, yellow, and lastly white. We have here a physical cause for the sense-inferiority of totally white animals which may account for their rarity in nature; for few, if any, wild animals are wholly white. The head, the face, or at least the muzzle or the nose, are generally black; the ears and eyes are also often black; and there is reason to believe that dark pigment is essential to good hearing, as it certainly is to perfect vision. We can therefore understand why white cats with blue eyes are so often deaf, a peculiarity we notice more readily than their deficiency of smell or taste.

[51] “Medico-Chirurgical Transactions,” vol. liii. (1870).

If, then, the prevalence of white coloration is generally associated with some deficiency in the acuteness of the most important senses, this colour becomes doubly dangerous; for it not only renders its possessor more conspicuous to its enemies, but at the same time makes it less ready in detecting the presence of danger. Hence, perhaps, the reason why white appears more frequently in islands, where competition is less severe and enemies less numerous and varied. Hence, also, a reason why _albinoism_, although freely occurring in captivity, never maintains itself in a wild state, while _melanism_ does. The peculiarity of some islands in having all their inhabitants of dusky colours (as the Galapagos) may also perhaps be explained on the same principles; for poisonous fruits may there abound which weed out all white- or light-coloured varieties, owing to their deficiency of smell and taste. We can hardly believe, however, that this would apply to white-coloured butterflies; and this may be a reason why the effect of an insular habitat is more marked in these insects than in birds or mammals.

It is even possible that this relation of sense-acuteness with colour may have had some influence on the development of the higher human races. If light tints of the skin were generally accompanied by some deficiency in the senses of smell, hearing, and vision, the white could never compete with the darker races so long as man was in a very low or savage condition, and wholly dependent for existence on the acuteness of his senses. But as the mental faculties became more fully developed and more important to his welfare than mere sense-acuteness, the lighter tints of skin and hair and eyes would cease to be disadvantageous whenever they were accompanied by superior brain-power. Such variations would then be preserved; and thus may have arisen the Xanthochroic race of mankind, in which we find a high development of intellect accompanied by a slight deficiency in the acuteness of the senses as compared with the darker forms.

_Relations of Insular Plants and Insects._--I have now to ask your attention to a few remarks on the peculiar relations of plants and insects as exhibited in islands.

Ever since Mr. Darwin showed the immense importance of insects in the fertilization of flowers, great attention has been paid to the subject, and the relation of these two very different classes of natural objects has been found to be more universal and more complex than could have been anticipated. Whole genera and families of plants have been so modified as, first to attract and then to be fertilized by, certain groups of insects; and this special adaptation seems in many cases to have determined the more or less wide range of the plants in question. It is also known that some species of plants can be fertilized only by particular species of insects; and the absence of these from any locality would necessarily prevent the continued existence of the plant in that area.

In this direction, I believe, will be found the clue to much of the peculiarity of the floras of oceanic islands; since the methods by which these have been stocked with plants and with insects will be often quite different. Many seeds are, no doubt, carried by oceanic currents, others probably by aquatic birds. Mr. H. N. Moseley informs me that the albatrosses, gulls, puffins, tropic birds and many others, nest inland, often amidst dense vegetation; and he believes they often carry seeds, attached to their feathers, from island to island for great distances. In the tropics they often nest on the mountains far inland, and may thus aid in the distribution even of mountain-plants. Insects, on the other hand, are mostly conveyed by aerial currents, especially by violent gales; and it may thus often happen that totally unrelated plants and insects may be brought together, in which case the former must often perish for want of suitable insects to fertilize them. This will, I think, account for the strangely fragmentary nature of these insular floras, and the great differences that often exist between those which are situated in the same ocean; as well as for the preponderance of certain orders and genera.