CHAPTER VI
Variation And Locality
In all discussions of the modes of Evolution the phenomena of Geographical Distribution have been admitted to be of paramount importance. First came the broad question, were the facts of distribution consistent with the Doctrine of Descent? I suppose all naturalists are now agreed that they are thus consistent, and that though some very curious and as yet inexplicable cases remain to be accounted for, the distribution of animal and plant life on the face of the earth is much what we might expect as a result of a process of descent with modification. Passing from this general admission to the more particular question whether the facts of distribution favour one special conception of the mode of progress of evolution rather than another, no agreement has yet been reached. One outstanding feature is hardly in dispute, namely that prolonged isolation is generally followed by greater or less change in the population isolated. Groups of individuals which from various causes are debarred from free intermixture with other groups almost always exhibit peculiarities, but on the other hand, cosmopolitan types which range over wide areas are on the whole uniform, or nearly so throughout their distribution. Examples of these two categories will be familiar to all naturalists. The barriers to intercourse may be seas, deserts, prairies, mountain-chains, or circumstances of a much less obvious character which isolate quite as effectually. The local unit is not necessarily an island, a district, or an area of special geological formation, but may, as every collector knows, be a valley, a pond, a creek, a "bank" in the sea, a clump of trees, a group of rocks in a bay, or a particular patch of ground on a mountain side. All the great groups provide examples of such specially isolated forms. The botanist knows them well; the conchologist, the entomologist, the ornithologist and the student of marine life are all equally aware that special varieties or special species come from special places and from nowhere else. In one remarkable case the season of appearance plainly acts as the isolating barrier. Tephrosia bistortata is a small Geometrid moth which has two broods, appearing in March and July respectively. It is closely allied to T. crepuscularia which emerges in May and June. From the fact that occasional specimens cannot be quite certainly referred to one or other of the two, many have held that the two are one species. Nevertheless, in general they present distinctions which are plain enough. Some localities have one form only, but in several woods they co-exist. Experiment has shown that the two can be crossed, and that the cross-breds can breed inter se and with at least one of the parent stocks.[1] Some diminution in fertility was observed, but perhaps not more than is commonly encountered when wild forms are bred in captivity. In such a case it can scarcely be doubted that the distinctness of the two forms in the places where they co-exist is maintained by the seasonal isolation.
Just as the consequences of isolation are to be seen in the most different forms of life so may they also affect the most diverse features of organisation, such as size, colour, sculpture, shape, or number of parts. In the Sloth (Choloepus) the geographical races differ in the number of cervical vertebrae—or in other words, in the distribution of vertebral differentiation. The geographical races of Cistudo differ in the number of claws and phalanges.[2]
In Shetland, the males of Hepialus humuli (the Ghost Moth) are not sharply differentiated in colour from the females, as they are elsewhere, but in varying degrees resemble them.[3] No such males are found in other localities, and even in the other Scottish islands they are normal. In the island of Waigiu the converse phenomenon has been observed in Phalanger maculatus. Generally the male is spotted with white, and the female is without spots, but in Waigiu the females are spotted like the males.[4]
The following striking illustration was pointed out to me by Dr. W. D. Miller. Euphonia elegantissima as it occurs in Mexico and Central America has the two sexes very distinct from each other. The male has the lower parts orange and the upper parts a dark indigo blue, with a bright turquoise-blue head and neck. The female, except for the head, is of a bright olive green. A form in which the sexes are similarly differentiated exists in Porto Rico and is known as E. Sclateri. But in many of the other West Indian islands the representative "species" (E. flavifrons) has the two sexes closely resembling the female of E. elegantissima. This form is found in Antigua, Barbados, St. Vincent, and Guadeloupe, from which localities the British Museum has specimens. All three so-called species are very much alike otherwise.
In the genus Pyrrhulagra (Loxigilla) to which Mr. Outram Bangs called my attention, several distinct and alternative possibilities occur. The genus has many local species occurring on the various West Indian islands. These species are characterized by differences in size, colour, and the shape of the bill. The colours have a narrow range, being black or greyish, with or without chestnut marks about the head and throat. In most of the islands the males are in general colour a full black, and the females are distinctly grey. They are thus found in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In Porto Rico we meet the peculiarity that the hens are almost as black as the males (Ridgway describes the black of the hens as slightly less intense). This form is called portoricensis. A larger type, known as grandis, similarly coloured, inhabits St. Kitt's. Then, on the contrary, in Barbados, both sexes are a dull blackish grey, like the hens of the Lesser Antilles in general.
The local species of Agelaius show similarly capricious distinctions. A. phoeniceus is a widely spread species, found over a great part of North America. The male is black with red-orange bars on the wings, but the female is somewhat thrush-like in colour. In the island of Porto Rico there is a form called xanthomus, in which both sexes are like the males of the mainland. A similar species called humeralis, also with both sexes male-like, lives in Cuba. The island of Cuba, curiously enough, has also a distinct species named assimilis, in which the female is a dull black all over, though the male is like the mainland type.
So also may local races differ in respect of variability. Argynnis paphia, the Silver Washed Fritillary, through a great part of its distribution has only one female form. In the English New Forest a second female form, valesina, co-exists with the ordinary paphia female. But in the southern valleys of the Alps the valesina female is much the commoner of the two, and indeed in some localities where the species is abundant, I have seen no paphia females in many days collecting.
The beetle Gonioctena variabilis furnishes an illustration of a comparable phenomenon affecting the male sex. In 1894 and 1895 I studied the curious colour variations of this species especially in the neighbourhood of Granada, and Mr. Doncaster ten years later repeated the observations on the same ground, and also collected the insect in other places in the south of Spain. The distinctions are not easy to give in words and the reader is referred to the colour plate accompanying my paper.[5] The essential fact is that the males commonly have the elytra red with black spots and the females for the most part have greenish grey elytra with black stripes. In some localities a large minority of males closely resemble the female type, being identical in colour and then only distinguishable by structural differences. In two Granada localities I found the proportion of such males quite different. In the Darro valley about 38 per cent. (in 718) were of this feminine type, but on the hills some 300 feet above only 19 per cent. (in 3,230) were like the females. At Castillejo, not far from Toledo I found no such male in 75 specimens.