On this evidence I have regarded the case as one in which there is no good evidence of segregation and as conforming most nearly with the conventional view of gradual transition in response to climatic influences. Such influence must however be indirect; for I reared five generations of the northern type in England, and these, though they included several abnormal-looking specimens in the last generation and then died out, did not show any noticeable change from the fulvous colour of the wild type. Merrifield[8] also found that heat applied to pupae of the northern type produced no approach to the southern type.
Looking at the facts now in the light of more experience it seems to me just possible that the case may be one in which, as in Nilson-Ehle's Wheats, the dominant differs from the recessive in having two pairs of factors with similar effects. The fulvous type for example may have two or more elements in separate pairs which together produce the full effect, and the intermediate may have one of these. If this were so, some segregation should of course eventually be observable, but the proportion of the various fulvous and fulvous-intermediate individuals would be large, and the reappearance of actual representatives of the northern type might be rare. I admit that this is a somewhat strained interpretation of the facts, and as yet it is not entitled to serious consideration. Nevertheless I am led to form some such expectation partly from the great difficulty in the way of any other, partly from the evidence of the small mixed sample found at Preuilly and partly from the statements given by Oberthür. There are moreover other features in the general distribution of the species which make it improbable that the dependence on climate can after all be so close. Published lists are unfortunately of little use in deciding which form occurs at a particular place, because, since the name Meone has ceased to be used for the southern form, there is no complete unanimity among authors as to the application of the names egeria and egerides, and unless more particulars are given, either name may be used for either form. Besides this, difficulty arises from the fact that the intermediate type is not generally distinguished at all, and English collectors finding it, may easily record it as the southern type. From Staudinger's note on the distribution, I gather that he, on the contrary, reckoned the intermediate with the northern type, as do the Speyers also. The late Mr. J. W. Tutt was careful to distinguish the three forms and has left several useful records. Easy therefore as it might seem to be to make out the distribution of such a familiar insect in its various modifications, there are serious practical difficulties, and until long series are brought together with this special object in view many obscurities will remain.
With only the series from England, the west of France, and Spain before one it would be easy to regard the successive series of tones as a fair measure of climate; the brighter the colour, the hotter might one expect the locality to be. Such rough correspondence is often to be observed in butterflies and birds. It becomes impossible to take these simple views in the light of more complete knowledge. Beginning with France the fulvous egeria occupies the lower valley of the Rhone, probably from well above Lyon, though I have no exact information respecting the country above Avignon. According to Speyer it also takes the department of Lozère. The same authority says that Puy-de-Dôme has "egeria," meaning perhaps the intermediate form, with the fulvous form much less commonly. Next comes the curious fact that though the Lower Rhone (Avignon, Tarascon, Nîmes) has the true fulvous form, Hyères, Cannes, Grasse, Nice, Digne, and Alassio have the intermediate. Savoy has the intermediate (Chambéry) and even egerides perhaps, though in the same latitude on the west of France there is nothing but the fulvous type. At Chalseul and Besançon (Doubs) the ordinary northern type is found. Switzerland generally, I believe, has the northern type, but Staudinger gives egeria for Valais and the intermediate occurs in Vaud.[9] The south side of the Alps has probably colonies of the pale egerides, and of intermediates. Orta, with a very hot summer, has the English type (Tutt, Ent. Rec., XII, 1900, p. 328). Locarno has the intermediate (ibid., XV, 1903, p. 321). North Italy in general and western Piedmont have the intermediate; but further south egeria begins, at what region I do not know. Speyer gives on his own authority the remarkable statement that at Florence both extremes occur, but chiefly intermediates between the two. Mr. R. Verity however kindly informs me that in his experience this is not so, and that neither the real southern type nor the northern occur there. Sardinia, Sicily, Crete all have the southern type. Greece probably has various types. Staudinger (Hor. Ross., VII, 1870, p. 78) says intermediates resembling Nice types common everywhere, but from "Greece" the British Museum has a series that would pass for English specimens; and the same type occurs near Constantinople. The island of Corfu has a pale intermediate, distinct from egerides but approaching it. In Roumania all three forms are recorded from various places: egeria in the Dobrutscha; not quite typical (presumably an intermediate) at Bukharest; intermediate in various mountainous localities as well as in Macedonia and Dalmatia; but egerides in Azuga at about 3,000 feet.[10] Hungary has the true egerides also. (Cf. Caradja, Deut. Ent. Zt., IX, p. 58.) Mathew records the same from Gallipoli (E. M. M., 1881, p. 95). Staudinger does not distinguish the intermediates from the northern, but he gives "egerides" for Armenia and Fergana (Central Asia). As against the mere proximity of a great mountain chain being the influence which keeps the Riviera population intermediate may be mentioned the fact that the northern foothills of the Pyrenees have the pure southern type, and the climate of Cambo must surely be far cooler than that of Nice. The exact locality of the Greek specimens is not given, but there can be no part of Greece which is not much hotter in summer than Brittany, or Calvados, which have the intermediate, not the English type.
In face of these facts it can scarcely be maintained that average temperature is the efficient cause of the particular tone of colour which the butterfly shows in a given region. Nevertheless it is clear that climate counts for much in determining the distribution. It is noticeable that though the pale egerides can be established in a warm climate we never find egeria in cold climates, and even the intermediate is not found in places that have a hard winter. I suspect that the distribution of the broods through the year and the condition of the animal at the onset of hard frost are features which really determine whether a strain can live in a particular place or not. Though the truth of the suggestion cannot be tested by experiments in captivity, which at once introduce disturbances, I incline to the idea that egeria has not got the right periodicity for northern climates. If it could arrange its life so that the population consisted either of young larvae, or perhaps of thoroughly formed pupae[11] at the onset of winter, it might, for any obvious reason to the contrary, be able to live in England. It is irregularly "polyvoltine," as the silk-worm breeders say, and as soon as a little warmth encourages it, a new generation starts into being, which if the frost comes at an untimely moment, is immediately destroyed. Many species are continually throwing off individuals which feed up fast[12] and emerge at once if the temperature permits, and I imagine a species of Satyrid wholly or largely represented by such individuals could scarcely survive in a country which had a hard winter. For such a climate some definite periodicity in the appearance of the broods may well be indispensable. But assuming that egeria is cut off from cold climates for such a reason, there is nothing yet to connect these habits with the fulvous colour, and until breeding can be carried out on a satisfactory scale there is no more to be said.
From time to time records appear of individual specimens more or less fulvous being caught in southern England, especially in the New Forest.[13] It would be interesting to know what offspring such individuals might produce. From the evidence now given some notion both of the strength and the weakness of the case considered as one of continuous climatic variation can be formed. I know no other equally satisfactory. Whether or not definite mixture of the intermediates with either of the extremes will be proved to occur, the case differs materially from those considered in the last chapter in the fact that at all events there is no general overlapping of forms. In a species so little given to wandering, overlapping could indeed scarcely be expected to occur. It is this circumstance which makes the species preeminently suitable as a subject for the study of climatic influences, and I trust that entomologists with the right opportunities may be disposed to explore the facts further.
Just as many species, like egeria, have varieties which can be regarded as adapted to northern and southern regions, so there are also several which have lowland and Alpine forms quite distinct from each other. Every such case presents an example of the problem we have been considering. As the collector passes from the plains to the Alpine region, how will he find the transition from one form to the other effected? Does the lowland form give place to the Alpine form suddenly, with a region in which the two are mixed, or will he find a zone inhabited by an intermediate population? I have spent a good deal of time examining the facts in the case of Pieris napi and its Alpine female variety bryoniae, and though there are many complications which still have to be cleared up, no doubt is possible as to the main lines of the answer. If in any valley in the Alps inhabited by both napi and bryoniae the collector catches every specimen he can, beginning at the bottom and working up to 7,000 feet, he will at first get nothing but napi. At about 2,500 feet, he may catch an occasional bryoniae flying with the napi. After 3,000 feet napi usually ceases, and only bryoniae are found. As an exception a colony of napi may be met with at much greater heights. I once found them in numbers at about 6,000 feet.[14] Not only were they free from any trace of modification in the direction of bryoniae, but they were of the thoroughly southern type of napi, being a late brood of that large and very pale kind (meridionalis) almost destitute both of dark veining above and of green veining below, which are common on the shores of Lago Maggiore and in other hot southern localities. Not far off at the same level were typical bryoniae in fair abundance. Occasionally an intermediate may be met with. I have taken a few, for example, at Macugnaga and at Fobello. These, however, in my experience are rarities in the Alps. Fleck[15] gives notes on the distribution in Roumania which shows the same state of things. The lowland form is not transformed though found at great heights, and at Azuga (nearly 3,000 feet) bryoniae occurs with only occasional "flavescens," viz., intermediates of the second brood.
If this were all the evidence we should be satisfied that the lowland and Alpine types keep practically distinct, overlapping occasionally, but rarely interbreeding. The problem would remain, how is the distinctness of the two types maintained in the region of overlapping? Nowadays, I suppose, we should incline to answer this question by reference to segregation, and perhaps by an appeal to selective mating. The suggestion that segregation does take place is certainly true to some extent. There are, however, difficulties in the way, and the whole subject is one of great complexity. My own experiments were made in pre-Mendelian times and were not arranged with the simplicity which we now know to be essential. The results are neither extensive enough nor clear enough to settle the many collateral questions which have to be considered, and the work ought to be done again. Nevertheless, some notes of the observations may have a suggestive value.
When I began, I did not sufficiently appreciate that the "napi" group, omitting the North American forms, and the Asiatic representatives, has at least three chief types in western Europe. The differences we have to deal with are manifested by the females only, so in this account particulars as to the males are omitted for the most part. These are (1) our own British napi; (2) the form found in the south, from the Loire downwards, and in the Italian Alps, which I think may be spoken of as meridionalis; (3) bryoniae, which is a form clearly recognizable in the female only, and is found only in the arctic regions and in the Alps above 2,500 feet. The first two have several broods, two, three, or more, according to opportunity, and the first brood is different from the later ones. In napi the markings on the upper surface are a dark grey but in meridionalis they are a pale silvery grey and much less extensive. In the later broods of napi there is much less general irroration of the veins, and the spots stand out as more defined and blacker. These differences vary greatly in degree of emphasis. In meridionalis the later broods are entirely different from the first. Instead of having silvery markings they have the ground colour quite white, with the spots large and a full black. On the under side of the hind wings the usual green veins are almost absent, and I have seen individuals which could scarcely be distinguished from rapae. To these later broods the term napaeae is sometimes applied, but I here use meridionalis for the southern race in general as applicable to all broods.
The female bryoniae is totally unlike the others. The ground colour is a full yellow, and each nervure is thickly irrorated with a brown pigment often spreading so far as to hide the ground almost entirely in the fore-wings. The males corresponding with these females are not certainly distinguishable from those of our own napi. Both sexes have the green veining of the underside of the hind wing fully developed, rather more than is usual in the lowland races, but this is not really diagnostic of the variety. The first serious difficulty arises in regard to the second brood of bryoniae. It is stated that there is only one brood,[16] but I feel fairly sure that a second brood is sometimes produced, and that the females with a yellow ground and diminished irroration of the veins, not very uncommon in the Italian Alps in July to August, are generally representatives of it. Such insects would of course be classed with bryoniae in collections.
My experiments began with eggs of true bryoniae females caught at about 2,500 feet early in July. These emerged in August-September as intermediates with yellow ground and about half as much black on the upper surface as bryoniae. They are exactly like the intermediates usually found in nature and in the light of later experience I regard them as natural F1 forms, and I think the mothers had been fertilised by napi males, though I admit that in view of the rarity of natural intermediates there is a difficulty in this suggestion. Three of these females were mated with males raised from thorough meridionalis females, and three families were produced. Two of them showed distinct evidence of segregation, some being yellow and some white with various intergrades, some being no blacker than meridionalis and some ranging up to a dark intermediate type. Part emerged in the same autumn; and part overwintered, emerging as the spring meridionalis or as the peculiar type which I afterwards learnt to know as the spring F1 form. The distinctions were fairly sharp between the several forms. But the offspring of the third female gave a series practically continuous from meridionalis to the F1 type. The work of subsequent years gave results similarly irregular which could only be described adequately at great length. The outcome may however be summed up in the statement that there is evidence that both the yellow ground and the dark veining are due to factors, but that there are several of these and that imperfect segregation is not uncommon, producing various reduction-stages. The yellow ground may be due to one factor, and the several shades may be the result of irregularities in dominance, but the black markings when fully developed cannot I think be the result of less than three factors, one for the basal darkening, one for general irroration, and one for the margins. Probably also the enlargement of the spots is produced by a fourth factor.