| C. indicus | C. affinis |
| Mantle drab brown-chestnut. | Dark olive-green. |
| Breast chestnut. | Dull purple brown. |
| Throat purplish, streaked with white. | Purple, streaked with blue. |
| Upper tail-coverts indigo. | Turquoise. |
The wings are the same in both. In the provinces of Nepal, Sikhim, and Darjiling the two species coexist, with the result that intergrades have been frequently recorded. The line of intergradation extends to the coast, and birds showing various combinations of the two types from the Calcutta district exist in collections.[12] The case is interesting inasmuch as like that of Quiscalus it shows a series of combinations of various metallic colours. Some of these are probably evoked by the development of pigment behind striations or other interferences already existing, but in the present state of knowledge it would be quite impossible to suggest what the actual factors producing these appearances may be.
There are, naturally, many other cases among birds which are suspected of being in reality comparable, but in most of them the evidence is still inadequate. Among Lepidoptera also there are a few of these; perhaps the most striking is that of Basilarchia "proserpina."[13] The genus is well known to European collectors under the name Limenitis, of which we in England have one species, L. sibylla, the "White Admiral." A species very like sibylla in general appearance is common in the northern parts of the United States, ranging through Canada and Northern New England, but rarely south of Boston. This species has the conspicuous white bands across both wings like our sibylla.
There is also a more Southern type known as astyanax, which is very different in its appearance, being without the white bands and having a broad irroration of blue scales on the posterior border of the hind wings. The two are so distinct that one would not be tempted to suspect any very close relation between them. In its distribution astyanax is described by Field as replacing arthemis south of latitude 42°. About Boston it is much more common than arthemis.
The two forms encroach but little on each other's territory, but where they do coexist, a third form, known as proserpina, is found which is almost intermediate, with the white bands much reduced. There is now no doubt that this proserpina is a heterozygous form, resulting from a combination of the characters of arthemis and astyanax. Field succeeded in rearing a brood of 16 from a proserpina mother caught wild which laid 31 eggs, and of these, nine (five males, four females) resembled the mother, being proserpina, and seven (four males, three females) were arthemis. There can be no question therefore that the mother had been fertilised by a male arthemis and that no-white-band is a factor partially dominant over the white band. Another point of interest which Field observed was that the proserpina female refused to lay on birch, poplar or willow, but accepted wild cherry (Prunus serotina) a species on which astyanax can live, though that tree is not known to be eaten by arthemis. Incidentally also the observations show that sterility cannot be supposed to be the bar which maintains the distinctness of arthemis and astyanax.
In this connection Papilio oregonia and bairdii should be mentioned.[14] P. oregonia is one of the numerous forms like machaon, but rather paler. It is a northern insect, inhabiting British Colombia east of the Cascade Range, and reaching to Colorado. P. bairdii is a much darker butterfly, representing the asterias group of the genus Papilio. Like asterias it has the abdomen spotted at the sides, not banded as in the machaon group. It belongs to Arizona and Utah extending into Colorado. From Colorado the form brucei is described, more or less intermediate, like bairdii but with the abdomen banded as in oregonia. W. H. Edwards records the results of rearing the offspring of the bairdii-like and of the oregonia-like mothers. Each was found able to have offspring of both kinds, that is to say, bairdii females gave both forms, and oregonia females gave both forms. It is not possible to say which is dominant, since the fathers were unknown. On general grounds one may expect that the bairdii form will be found to dominate, but this is quite doubtful.
From this particular discussion I omit reference to those examples in which the permanently established types are obviously associated with special conditions of life. Where considerable climatic differences exist between localities, or when we pass from South to North, or from the plains into Alpine levels we often find that in correspondence with the change of climate there is a change in the characteristics of a species common to both. When I say "species" in such a connection I am obviously using the term in the inclusive sense. Some would prefer to say that in the two sets of conditions two representative species exist. Whichever expression be preferred it is plain that such examples present another phase of the problem we have been just considering, and in them also we have an opportunity of observing the consequences of the overlap of two closely related types, but there are advantages in considering them separately. In the examples hitherto given, with the possible exception of the Papilios,[15] the two fixed types severally range over so extensive a region that it may fairly be supposed that in the different parts they are subject to considerable diversities of climate. There is no outstanding difference that we know distinguishing the habitats of the two forms; but in comparing Alpine with Lowland forms, or essentially northern with essentially southern forms we do know an external circumstance, temperature, that may reasonably be supposed to have an influence, direct or indirect, on the population.