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

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СКАЧАТЬ organs differing in the two generations. According to Claus,43 “we can scarcely find any other explanation of the mode of origination of heterogenesis than the gradual and slow advantageous adaptation of the organization to important varying conditions of life” – a judgment in which this author is certainly correct. In all such cases the change does not affect unimportant characters, as it does in butterflies, but parts of biological or physiological value; and we cannot, therefore, consider such changes to have originated through the direct action of altered conditions of life, but indirectly through natural selection or adaptation.

      Thus, the difference between seasonal dimorphism and the other known cases of heterogenesis consists in the secondary form in which the species appears in the former originating through the direct action of external conditions, whilst in the latter this form most probably originates through the indirect action of such influences. The first half of the foregoing proposition is alone capable of provisional proof, but it is in the highest degree probable that the latter half is also correct. Naturally we cannot say to what extent the direct action of external conditions plays also a part in true heterogenesis, as there have been as yet no experiments made on its origin. That direct action, working to a certain extent co-operatively, plays only a secondary part, while the chief cause of the change is to be found in adaptation, no one can doubt who keeps in view, for instance, the mode of propagation discovered by Leuckart in Ascaris nigrovenosa. In this worm, the one generation lives free in the water, and the other generation inhabits the lungs of frogs, the two generations differing from one another in size of body and structure of internal organs to an extent only possible with the true Nematoda.

      To prevent possible misunderstanding, let it be finally noted – even if superfluous – that the changes causing the diversity of the two generations in seasonal dimorphism and heterogenesis are not of such a nature that the value of different “specific characters” can be attached to them. Distinctly defined specific characters are well known not to occur generally, and it would therefore be erroneous to attach but little value to the differences in seasonal dimorphism because these chiefly consist in the colouring and marking of the wings. The question here under consideration is not whether two animal forms have the value of species or of mere varieties – a question which can never be decided, since the reply always depends upon individual opinion of the value of the distinctions in question, and the idea of both species and varieties is moreover purely conventional. The question is, rather, whether the distinguishing characters possess an equal constancy – that is, whether they are transmitted with the same force and accuracy to all individuals; and whether they occur, therefore, in such a manner that they can be practically employed as specific characters. With respect to this, it cannot be doubtful for a moment that the colouring and marking of a butterfly possess exactly the same value as the constant characters in any other group of animals, such as the palate-folds in mice, the structure of the teeth in mammals, the number and form of the wing and tail feathers in birds, &c. We have but to remember with what wonderful constancy often the most minute details of marking are transmitted in butterflies. The systematist frequently distinguishes between two nearly allied species, as for instance in the Lycænidæ, chiefly by the position of certain insignificant black spots on the under side of the wing (P. Alexis female, and P. Agestis); and this diagnosis proves sufficient, since P. Alexis, which has the spots in a straight row, has a different caterpillar to P. Agestis, in which the central spot is nearer the base of the hind wing!

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      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

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, СКАЧАТЬ