Studies in the Theory of Descent, Volume I. Weismann August

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СКАЧАТЬ Napi in its winter form is the first secondary form gradually produced through a warm climate, then it would be impossible ever to breed the ordinary form Napi from pupæ of Bryoniæ by the action of warmth, since the form of the species now predominant must have come into existence only by a cumulative action exerted on numerous generations, and not per saltum.

      The experiment was made in the following manner: In the first part of June I caught a female of Bryoniæ in a secluded Alpine valley, and placed her in a capacious breeding-cage, where she flew about among the flowers, and laid more than a hundred eggs on the ordinary cabbage. Although the caterpillars in the free state feed upon another plant unknown to me, they readily ate the cabbage, grew rapidly, and pupated at the end of July. I then brought the pupæ into a hothouse in which the temperature fluctuated between 12° and 24° R.; but, in spite of this high temperature, and – what is certainly of more special importance – notwithstanding the want of cooling at night, only one butterfly emerged the same summer, and that a male, which, from certain minute characteristic markings, could be safely identified as var. Bryoniæ. The other pupæ hibernated in the heated room, and produced, from the end of January to the beginning of June, 28 butterflies, all of which were exquisite Bryoniæ.

      Experiment thus confirmed the view that Bryoniæ is the parent-form of Napi, and the description hitherto given by systematists ought therefore properly to be reversed. Pieris Bryoniæ should be elevated to the rank of a species, and the ordinary winter and summer forms should be designated as vars. Napi and Napeæ. Still I should not like to take it upon myself to increase the endless confusion in the synonomy of butterflies. In a certain sense, it is also quite correct to describe the form Bryoniæ as a climatic variety, for it is, in fact, established, if not produced, by climate, by which agency it is likewise preserved; only it is not a secondary, but the primary, climatic variety of Napi. In this sense most species might probably be described as climatic varieties, inasmuch as under the influence of another climate they would gradually acquire new characters, whilst, under the influence of the climate now prevailing in their habitats, they have, to a certain extent, acquired and preserved their present form.

      The var. Bryoniæ is, however, of quite special interest, since it makes clear the relation which exists between climatic variation and seasonal dimorphism, as will be proved in the next section. The correctness of the present theory must first here be submitted to further proof.

      It has been shown that the secondary forms of seasonally dimorphic butterflies do not all possess the tendency to revert in the same degree, but that this tendency rather varies with each individual. As the return to the primary form is synonymous with the relinquishing of the secondary, the greater tendency to revert is thus synonymous with the greater tendency to relinquish the secondary form, but this again is equivalent to a lesser stability of the latter; it must consequently be concluded that the individuals of a species are very differently influenced by climatic change, so that with some the new form must become sooner established than with others. From this a variability of the generation concerned must necessarily ensue, i.e., the individuals of the summer generation must differ more in colour and marking than is the case with those of the winter generation. If the theory is correct, the summer generations should be more variable than the winter generations – at least, so long as the greatest possible equalization of individual variations has not occurred through the continued action of warmth, combined with the constant crossing of individuals which have become changed in different degrees. Here also the theory is fully in accord with facts.

      In A. Levana the Levana form is decidedly more constant than the Prorsa form. The first is, to a slight extent, sexually dimorphic, the female being light and the male dark-coloured. If we take into consideration this difference between the sexes, which also occurs to a still smaller extent in the Prorsa form, the foregoing statement will be found correct, viz. that the Levana form varies but little, and in all cases considerably less than the Prorsa form, in which the greatest differences occur in the yellow stripes and in the disappearance of the black spots on the white band of the hind wing, these black spots being persistent Levana markings. It is, in fact, difficult to find two perfectly similar individuals of the Prorsa form. It must, moreover, be considered that the Levana marking, being the more complicated, would the more readily show variation. Precisely the same thing occurs in Pieris Napi, in which also the var. Æstiva is considerably more variable than the var. Vernalis. From the behaviour of the var. Bryoniæ, on the other hand, which I regard as the parent-form, one might be tempted to raise an objection to the theory; for this form is well known to be extraordinarily variable in colour and marking, both in the Alps and Jura, where it is met with at the greatest altitudes. According to the theory, Bryoniæ should be less variable than the winter form of the lowlands, because it is the older, and should therefore be the more constant in its characters. It must not be forgotten, however, that the variability of a species may not only originate in the one familiar manner of unequal response of the individual to the action of varying exciting causes, but also by the crossing of two varieties separately established in adjacent districts and subsequently brought into contact. In the Alps and Jura the ordinary form of Napi swarms everywhere from the plains towards the habitats of Bryoniæ, so that a crossing of the two forms may occasionally, or even frequently, take place; and it is not astonishing if in some places (Meiringen, for example) a perfect series of intermediate forms between Napi and Bryoniæ is met with. That crossing is the cause of the great variability of Bryoniæ in the Alpine districts, is proved by the fact that in the Polar regions this form “is by no means so variable as in the Alps, but, judging from about forty to fifty Norwegian specimens, is rather constant.” My friend, Dr. Staudinger, who has twice spent the summer in Lapland, thus writes in reply to my question. A crossing with Napi cannot there take place, as this form is never met with, so that the ancient parent-form Bryoniæ has been able to preserve its original constancy. In this case also the facts thus accord with the requirements of the theory.

      II. Seasonal Dimorphism and Climatic Variation

      If, as I have attempted to show, seasonal dimorphism originates through the slow operation of a changed summer climate, then is this phenomenon nothing else than the splitting up of a species into two climatic varieties in the same district, and we may expect to find various connexions between ordinary simple climatic variation and seasonal dimorphism. Cases indeed occur in which seasonal dimorphism and climatic variation pass into each other, and are interwoven in such a manner that the insight into the origin and nature of seasonal dimorphism gained experimentally finds confirmation. Before I go more closely into this subject, however, it is necessary to come to an understanding as to the conception “climatic variation,” for this term is often very arbitrarily applied to quite dissimilar phenomena.

      According to my view there should be a sharp distinction made between climatic and local varieties. The former should comprehend only such cases as originate through the direct action of climatic influences; while under the general designation of “local forms,” should be comprised all variations which have their origin in other causes – such, for example, as in the indirect action of the external conditions of life, or in circumstances which do not owe their present existence to climate and external conditions, but rather to those geological changes which produce isolation. Thus, for instance, ancient species elsewhere long extinct might be preserved in certain parts of the earth by the protecting influence of isolation, whilst others which immigrated in a state of variability might become transformed into local varieties in such regions through the action of ‘amixia,’24 i.e. by not being allowed to cross with their companion forms existing in the other portions of their habitat. In single cases it may be difficult, or for the present impossible, to decide whether we have before us a climatic form, or a local form arising from other causes; but for this very reason we should be cautious in defining climatic variation.

      The statement that climatic forms, in the true sense of the word, do exist is well known to me, and has been made unhesitatingly by all zoologists; СКАЧАТЬ