Another form in which I tried to investigate the same problem is Coenonympha arcania, which has one Alpine form known as darwiniana, and another, satyrion. In calling satyrion a form of arcania I follow Staudinger and other authorities, but I have never been quite satisfied that it should be so regarded. The differences between arcania and darwiniana are essentially differences of degree; C. arcania occurs in places where there is cover, and reaches up the valleys usually as high as the mixed woods of deciduous trees, which is about 2,500 feet. The variety darwiniana, on the contrary, is an insect of treeless hillsides, and I regard it as a dwarf and possibly a stunted form. It would not greatly surprise me to find that with the application of good conditions arcania could be raised from darwiniana eggs, or that if arcania larvae were starved they might give rise to darwiniana butterflies. I have been unsuccessful in trying to rear the species, having lost the larvae by disease. Usually one does not catch arcania and darwiniana on the same ground, and as Festuca ovina—a typically hill-side grass—is a common food-plant of darwiniana there can be little doubt that arcania feeds on some other grass, probably woodland species. Colonies of arcania of varying size and brightness are commonly found, and though a sample of arcania, finely grown, from a warm Italian wood, presents a striking contrast with darwiniana from an Alpine pasture, one certainly may get samples which fill all the gradations. Generally the sample from a given locality is fairly homogeneous.
Of satyrion I have little personal experience. I only twice found it, namely at Zinal, and at Hallstatt in Austria, but it occurs at Zermatt, Arolla, and in several Swiss localities above 5,000 feet, and I understand that it is the typical Alpine form in the Engadine. With its darkened colour and reduced size it might well be expected to be a still further stunted form of darwiniana. Yet I have never found the one succeed to the other at the higher levels. If darwiniana appears when Alpine conditions are reached in a valley it will be met with up to the highest level at which such butterflies live. Tutt was of opinion that satyrion is a distinct species.[20] I once, at the top of the Vorderrheinthal caught a sample of darwiniana a few of which (males) were so dark and had the eye spots so poorly developed that they looked like transitions to satyrion. Otherwise I never found any such transitional forms and they are certainly exceptional. There is further a record[21] of satyrion having been taken flying with arcania. This was near Susa, at about 2,000 feet I infer. Mr. H. E. Page has similar specimens from Caud and from St. Anton (Arlberg). The females, however, both of mine and of Mr. Page's samples are a pale brown, quite unlike the females both of arcania and of the dark Zinal satyrion. The difficulty thus raised has not I think yet been considered by the authorities, and it is possible that the Alpine forms of arcania are in reality three, not two.
The evidence taken together suggests, I think, that darwiniana is related to arcania much as so many of the Alpine varieties of plants are to the well-developed individuals of the lower levels. I do not anticipate that factorial differences will be found in these insects, and it is by no means impossible that the distinctions between them are the direct consequences of altered conditions. The relations of arcania to satyrion are more doubtful, and in that case a factorial difference may at least be suspected.
The species of the genus Setina have Alpine forms which agree in possessing a characteristic extension of the black pigment to form radiating junctions between the spots on the wings. Speyer, who discussed the interrelations of these forms in detail,[22] lays stress on the absence of genuine transitional forms between aurita and the variety ramosa. Both are mountain insects but ramosa extends to levels higher than that at which aurita ceases, which is about 4,000 feet. The two forms are often found flying together. Speyer says that his brother searched diligently for transitional forms at the level of overlapping, but found none, so that at least they may be regarded as rare. The variety ramosa is not infrequent at much lower levels (e. g., Chiavenna, 1,020 feet; Reussthal, 1,500 feet) and extends as high as the permanent snows. In the British Museum collection, however, I have seen several that I should regard as transitional. Speyer perhaps would have classed as ramosa all in which the spots of the central field were united, and it is by no means unlikely that breeding would prove such individuals to be heterozygous.[23]
There can scarcely be a doubt that the distinction between aurita and ramosa is factorial, the radiate ramosa probably having the factor for striping. In support of this view may be mentioned the observation of Boisduval,[24] respecting a gynandromorphous individual, which was aurita male on one side, and ramosa female on the other. Speyer makes another excellent comment. He points out that the simple notion that the radiation is a mere extension of pigmentation consequent on the climate of the higher levels, will not fit the facts very easily, because the size of the spots varies greatly in aurita itself at any level, and lowland specimens may actually have more black confined to the spots alone than some ramosa possess on spots and lines combined.[25]
The two Salamanders, S. maculosa and its Alpine form atra, might not improbably furnish evidence bearing on the same problem. The two are of course very distinct, not merely in colour (maculosa being spotted with yellow or orange while atra is entirely black) but also in the mode of reproduction, a feature to which reference will be made in the next chapter. I cannot, however, find any evidence as to the overlapping of the two forms. S. atra occurs from about 3,000 feet or somewhat less, and reaches great elevations in the Eastern Alps, but I do not know if the two forms ever occur in the same localities. Leydig,[26] Boulenger,[27] and most modern authorities regard the two types as distinct species, but they are in any case closely allied, and it would be of interest to have exact knowledge of their geographical delimitations.
The reader who has considered the cases adduced will appreciate the difficulties which must be faced in any attempt to account for the facts in a rational way. As always in a problem of Evolution, two separate questions have to be answered. First how did the form under consideration come into existence, and secondly, how did it succeed in maintaining itself so as to become a race? The evidence from the local forms, though very far from giving complete answers to either of these questions definitely refutes the popular notion that a new race comes into existence by transformation of an older race. If a gradual mass-transformation of this kind took place we should certainly expect that when two types, nearly allied and capable of interbreeding, overlap each other in their geographical distribution, a normally intermediate population would exist. If each type can maintain itself, and if each came into existence by gradual transformation, then there must have been an intermediate capable of existing and maintaining itself as a population; and if this had ever been, surely in the region of overlapping, that intermediate population should continue. Especially should such a population be found when the two extreme types are adaptational forms and the region of overlap is a region of intermediate conditions. But of the examples we have examined there is only one, that of Pararge egeria and egerides, which can at all be so interpreted, and even in that case it is not impossible that more minute observation would reveal discontinuity between the extremes and the admittedly normal intermediate population. Granting provisionally however that this example, as it stands, is consistent with the conventional theory of evolution, I know not where we should look for another case equally good. When the distinctions are produced by direct influence of conditions operating during the lifetime of the individuals, examples of intermediate populations occupying the areas of intermediate conditions can no doubt be produced. Many turf-like Alpine plants, for instance, if protected from exposure and properly nourished can grow as large as those of the same species found in the valleys, and in the case of such quantitative effects, intermediate conditions can doubtless produce intermediate characters.
Even these examples however are not very abundant, and often the intermediate locality has not a form intermediate between those of the two extreme localities, but some third form distinct from either. This is the case for instance in the fauna of brackish waters. We are taught to believe that the fresh water fauna was evolved from the marine fauna, which it well may have been; but as students of Crustacea and Mollusca know familiarly, the brackish water forms are not as a rule intermediates between fresh water species and sea species, but more usually they are special forms belonging to the brackish waters, with the peculiar property that they can tolerate a great range of conditions, and live without ostensible variation in waters of most various compositions and densities, which very few marine or fresh water species are able to do.
Sometimes the distinction between local races, as in Rhamphocoelus passerinii and icteronotus may be regarded with confidence as due to one simple Mendelian factor possessed by one race and absent from the other, but I think, more often, as in Colaptes or in the varieties of Pieris napi, the existence of several distinct factors is to be inferred. As we have seen, the races of Colaptes show almost beyond doubt that in different areas at least three distinct factorial combinations can be perpetuated as races.
In the distribution of variability we find, I think, some hint as to the steps by which the phenomena under consideration have come to their present stage, and I am disposed to regard the facts so well attested in the case of our own melanic moths as a true indication of the process. Following this indication we should regard the change in the character of a population as beginning sporadically, by the appearance of varying individuals, possibly only one varying individual, in, it may be, one place only. As to why a variety should increase in numbers we have nothing but mere speculation to offer, and for the present we must simply recognise the fact that it may. That such survival and replacement may reasonably be taken as an indication that the replacing race has some superior power of holding its own I am quite disposed to admit. Nevertheless it seems in the highest degree unlikely that the outward and perceptible character or characters which we recognise as differentiating the race should be the actual features which contribute effectively to that result.