Studies in the Theory of Descent, Volume I

Before I proceed to the solution of this question it is indispensable that one point should be cleared up which has not been hitherto touched upon, but which must be settled before the problem can be formally stated in general terms. Before it can be asked whether larvæ and imagines have undergone a precisely parallel development, we must know whether unequal development is possible – whether there does not exist such an intimate structural relationship between the two stages that every change in one of these must bring about a change in the other. Were this the case, every change in the butterfly would cause a correlative change in the caterpillar, and vice versâ, so that an inequality of form-relationship between the larvæ on one hand and the imagines on the other would be inconceivable – systems based on the characters of the caterpillars would completely coincide with those based on the characters of the butterflies and we should arrive at a false conclusion if we attributed the phyletically parallel development of the two stages to the existence of an internal phyletic force, whilst it was only the known factor, correlation, which caused the equality of the course of development.

For these reasons it must first be established that the larva and imago are not respectively fixed in form, and the whole of the first section will therefore be devoted to proving that the two stages change independently of one another. Conclusions as to the causes of change will then be drawn, and these will corroborate from another side a subsequent inquiry as to the presence or absence of complete congruence in the two morphological systems. The two questions the answers to which will be successively attempted are by no means identical, although closely related, since it is quite conceivable that the first may be answered by there being no precise correlation of form, or only an extremely small correlation, between the caterpillar and the imago, whilst, at the same time, it would not be thereby decided whether the phyletic development of the two stages had kept pace uniformly or not. A perfect congruence of morphological relationships could only take place if transformations resulted from an internal power instead of external influences. The question: – Does there exist a fixed correlation of form between the two stages? must therefore be followed by another: – Do the form-relationships of the two stages coincide or not – has their phyletic development been uniform or not?

notes

1

A most minute and exact description of the newly hatched larva of Chionobas Aëllo is given by the American entomologist, Samuel H. Scudder. Ann. Soc. Ent. de Belgique, xvi., 1873.

2

I am aware that this certainly cannot be said of philosophers like Lotze or Herbert Spencer; but these are at the same time both naturalists and philosophers.

3

“Über die Artrechte des Polyommatus Amyntas und Polysperchon.” Stett. ent. Zeit. 1849. Vol. x. p. 177–182. [In Kirby’s “Synonymic Catalogue of Diurnal Lepidoptera” Plebeius Amyntas is given as a synonym and P. Polysperchon as a var. of P. Argiades Pall. R.M.]

4

“Die Arten der Lepidopteren-Gattung Ino Leach, nebst einigen Vorbemerkungen über Localvarietäten.” Stett. ent. Zeit. 1862. Vol. xxiii. p. 342.

5

[Eng. ed. W. H. Edwards has since pointed out several beautiful cases of seasonal dimorphism in America. Thus Plebeius Pseudargiolus is the summer form of P. Violacea, and Phyciodes Tharos the summer form of P. Marcia. See Edwards’ “Butterflies of North America,” 1868–79.]

6

[Eng. ed. I learn by a written communication from Dr. Speyer that two Geometræ, Selenia Tetralunaria and S. Illunaria Hüb., are seasonally dimorphic. In both species the winter form is much larger and darker.] [Selenia Lunaria, S. Illustraria, and some species of Ephyra (E. Punctaria and E. Omicronaria) are likewise seasonally dimorphic. For remarks on the case of S. Illustraria see Dr. Knaggs in Ent. Mo. Mag., vol. iii. p. 238, and p. 256. Some observations on E. Punctaria were communicated to the Entomological Society of London by Professor Westwood in 1877, on the authority of Mr. B. G. Cole. See Proc. Ent. Soc. 1877, pp. vi, vii. R.M.]

7

[In 1860 Andrew Murray directed attention to the disguising colours of species which, like the Alpine hare, stoat, and ptarmigan, undergo seasonal variation of colour. See a paper “On the Disguises of Nature, being an inquiry into the laws which regulate external form and colour in plants and animals.” Edinb. New Phil. Journ., Jan. 1860. In 1873 I attempted to show that these and other cases of “variable protective colouring” could be fairly attributed to natural selection. See Proc. Zoo. Soc., Feb. 4th, 1873, pp. 153–162. R.M.]

8

[A phenomenon somewhat analogous to seasonal change of protecting colour does occur in some Lepidoptera, only the change, instead of occurring in the same individual, is displayed by the successive individuals of the same brood. See Dr. Wallace on Bombyx Cynthia, Trans. Ent. Soc. Vol. v. p. 485. R.M.]

9

“Über den Einfluss der Isolirung auf die Artbildung.” Leipzig, 1872, pp. 55–62.

10

[Mr. A. R. Wallace maintains that the obscurely coloured females of those butterflies which possess brightly coloured males have been rendered inconspicuous by natural selection, owing to the greater need of protection by the former sex. See “Contributions to the Theory of Natural Selection,” London, 1870, pp. 112–114. It is now generally admitted that the underside of butterflies has undergone protectional adaptation; and many cases of local variation in the colour of the underside of the wings, in accordance with the nature of the soil, &c., are known. See, for instance, Mr. D. G. Rutherford on the colour-varieties of Aterica Meleagris (Proc. Ent. Soc. 1878, p. xlii.), and Mr. J. Jenner Weir on a similar phenomenon in Hipparchia Semele (loc. cit. p. xlix.) R.M.]

11

[The fact that moths which, like the Geometræ, rest by day with the wings spread out, are protectively marked on the upper side, fully corroborates this statement. R.M.]

12

“Über die Einwirkung verschiedener, während der Entwicklungsperioden angewendeter Wärmegrade auf die Färbung und Zeichnung der Schmetterlinge.” A communication to the Society of Natural Science of Steiermark, 1864.

13

See Exp. 9, Appendix I (#P1_A1).

14

See Exp. 11, Appendix I (#P1_A1).

15

See Exps. 4, 9, and 11, Appendix I (#P1_A1).

16

It seems to me very necessary to have a word expressing whether a species produces one, two, or more generations in the year, and I have therefore coined the expression mono-, di-, and polygoneutic from γονεύω, I produce.

17

[Eng. ed. In the German edition, which appeared in 1874, I was not able to support this hypothesis by geographical data, and could then only ask the question “whether in the most northern portion of its area of distribution, appears in two or only in one generation?” This question is now answered by the Swedish Expedition to the Yenisei in 1876. Herr Philipp Trybom, one of the members of this expedition, observed A. Levana at the end of June and beginning of July, in the middle of Yenisei, in 60°-63° N. (Dagfjärilar från Yenisei in Översigt ap k. Vertensk. Akad. Förhandlingon, 1877, No. 6.) Trybom found Levana at Yenisk on June 23rd, at Worogova (61° 5´) on July 3rd, at Asinova (61° 25´) on July 4th, at Insarowa (62° 5´) on July 7th, and at Alinskaja (63° 25´) on July 9th. The butterflies were especially abundant at the beginning of June, and were all of the typical Levana form. Trybom expressly states, “we did not find a single specimen which differed perceptibly from Weismann’s Figs. 1 and 2 (#Plate_I) (‘Saison-Dimorphismus’ Taf. I.).”

The Swedish expedition soon left the Yenisei, and consequently was not able to decide by observations whether a second generation possessing the Prorsa form appeared later in the summer. Nevertheless, it may be stated with great probability that this is not the case. The districts in which Levana occurs on the Yenisei have about the same isotherm as Archangel or Haparanda, and therefore the same summer temperature. Dr. Staudinger, whose views I solicited, writes to me: – “In Finnmark (about 67° N.) I observed no species with two generations; even Polyommatus Phlæas, which occurs there, and which in Germany has always two, and in the south, perhaps, three generations, in Finnmark has only one generation. A second generation would be impossible, and this would also be the case with Levana in the middle of Yenisei. I certainly have Levana and Prorsa from the middle of Amur, but Levana flies there at the end of May, and the summers are very warm.” The middle of Amur lies, moreover, in 50° N. lat., and therefore 10°-13° south of the districts of the Yenisei mentioned.

It must thus be certainly admitted that on the Yenisei A. Levana occurs only in the Levana form, and that consequently this species is at the present time, in the northernmost portion of its area of distribution, in the same condition as that in which I conceive it to have been in mid Europe during the glacial period. It would be of the greatest interest to make experiments in breeding with this single-brooded Levana from the Yenisei, i.e., to attempt to change its offspring into the Prorsa form by the action of a high temperature. If this could not be accomplished it would furnish a confirmation of my hypothesis than which nothing more rigorous could be desired.]

18

See Exp. 10, Appendix I (#P1_A1).

19

When Dorfmeister remarks that hibernating pupæ which, at an early stage “were taken for development into a room, or not exposed to any cold, gave dwarfed, weakly and crippled,” or otherwise damaged butterflies, this is entirely attributable to the fact that this able entomologist had neglected to supply the necessary moisture to the warm air. By keeping pupæ over water I have always obtained very fine butterflies.

20

[For other remarkable cases of sexual dimorphism (not antigeny in the sense used by Mr. S. H. Scudder, Proc. Amer. Acad., vol. xii. 1877, pp. 150–158) see Wallace “On the Phenomena of Variation and Geographical Distribution, as illustrated by the Papilionidæ of the Malayan Region,” Trans. Linn. Soc., vol. xxv. 1865, pp. 5–10. R.M.]

21

[Eng. ed. Dimorphism of this kind has since been made known: the North American Limenitis Artemis and L. Proserpina are not two species, as was formerly believed, but only one. Edwards bred both forms from eggs of Proserpina. Both are single-brooded, and both have males and females. The two forms fly together, but L. Artemis is much more widely distributed, and more abundant than L. Proserpina. See “Butterflies of North America,” vol. ii.]

22

[Eng. ed. Edwards has since proved experimentally that by the application of ice a large proportion of the pupæ do indeed give rise to the var. Telamonides. He bred from eggs of Telamonides 122 pupæ, which, under natural conditions, would nearly all have given the var. Marcellus. After two months’ exposure to the low temperature there emerged from August 24th to October 16th, fifty butterflies, viz. twenty-two Telamonides, one intermediate form between Telamonides and Walshii, eight intermediate forms between Telamonides and Marcellus more nearly related to the former, six intermediate forms between Telamonides and Marcellus, but more closely resembling the latter, and thirteen Marcellus. Through various mishaps the action of the ice was not complete and equal. See the “Canadian Entomologist,” 1875, p. 228. In the newly discovered case of Phyciodes Tharos also, Edwards has succeeded in causing the brood from the winter form to revert, by the application of ice to this same form. See Appendix II (#P1_A2). for a résumé of Edwards’ experiments upon both Papilio Ajax and Phyciodes Tharos. R.M.]

23