NATURE 



141 



THURSDAY, JUNE 17, 1S80 



TWO DARWINIAN ESSAYS 

 Studies in the Theory 0/ Descent. By Dr. Aug. Weismann, 

 Professor in the University of Freiburg. Translated 

 and Edited by Raphael Meldola, F.C.S., Secretary of 

 the Entomological Society of London. Part I. On the 

 Seasonal Dimorphism of Butterflies, with Two Coloured 

 Plates. (London : Sampson Low, Marston, and Co., 

 1880.) 

 Degeneration. A Chapter in Dar-ii'inistn. By Prof. E. 

 Ray Lankester, F.R.S. NATURE Series. (Macmillan 

 and Co., 18S0.) 



THE first of Dr. Weismann's "Studies," of which Mr. 

 Meldola has given us an excellent translation, with 

 the author's latest notes and additions, is devoted to a 

 thorough examination of the well-known but hitherto little 

 understood phenomenon of the seasonal forms of butter- 

 flies. For the benefit of those unacquainted with ento- 

 mology we may state, that many butterflies have two, or 

 even three broods in a year. One brood appears in 

 spring, their larvae having fed during the preceding 

 autumn and passed the winter in the pupa state, while 

 the others appear later in the year, having passed rapidly 

 through all their transformations and thus never having 

 been exposed to the cold of winter. In most cases the 

 insects produced under these opposite conditions present 

 little or no perceptible dift'erence ; but in others there is 

 a constant variation, and sometimes this is so great that 

 the two forms have been described as distinct species. 

 The most remarkable case among European butterflies is 

 that of Araschnia prorsa, the winter or spring form of 

 which was formerly considered to be a distinct species 

 and named Araschnia levana. The two insects differ 

 considerably in both sexes, in markings, in colour, and 

 even in the form of the wings, so that till they were bred 

 and found to be alternate broods of the same species 

 (about the year 1 830) no one doubted their being altogether 

 distinct. 



In order to learn something of the origin and nature of 

 this curious phenomenon Dr. Weismann has for many 

 years carried on a variety of experiments, breeding the 

 species in large numbers and subjecting the pup;i: to 

 artificial heat or cold for the purpose of hastening or 

 retarding the transformation. The result of these experi- 

 ments is, that by subjecting the summer brood to severe 

 artificial cold in the pupa state, it may be made to produce 

 perfect insects the great majority of which are of the 

 winter form ; but, on the other hand, no change of con- 

 ditions that have yet been tried have any effect in changing 

 the winter to the summer form. Taking this result in 

 connection with the fact that in high latitudes where there 

 is only one brood a year it is always the winter form. Dr. 

 Weismann was led to the hypothesis that this winter 

 form was the original type of the species, and that the 

 summer form has been produced gradually, since the 

 glacial epoch, by the summer becoming longer and thus 

 admitting of the production of a second or summer brood. 

 This explains why the production of the winter form {A. 

 levana) from summer larvae is easy, it being a reversion 

 Vol. XXII. — No. 555 



to the ancestral type; while the production of the summer 

 form {A. prorsci) from autumnal larvae is impossible, 

 because that form is the result of gradual development ; 

 and processes of development which have taken thousands 

 of years to bring about cannot be artificially reproduced in 

 a single season. 



This hypothesis was supported by experiments with 

 another two-brooded species, Picris napi, with similar 

 results, the winter form being produced with certainty by 

 the application of cold to summer pupx ; and Mr. 

 Edwards, in America, has made similar experiments with 

 the various forms of Papilii aja.v, finding that the summer 

 broods can be changed into the winter form by the appli- 

 cation of cold, while the winter broods can never be made 

 to assume the summer form by hastening the process of 

 transformation. In the Arctic regions and in the high 

 Alps there is only one form of Picris napi, which very 

 closely resembles the winter form of the rest of Europe, 

 and this could never be the least changed by rapidly 

 developing the pups under the influence of heat. 



Another curious case is that of one of the Lycaanida; 

 (Plebeins agestis) which exhibits three forms, which may be 

 designated as A, B, and C. The first tv/o, A and B, are 

 alternate broods (winter and summer) in Germany, while 

 in Italy the corresponding forms are B and C, so that B 

 is the summer form in Germany and the winter form in 

 Italy. Here we see climatic varieties in process of 

 formation in a very curious way. 



That temperature during the pupa stage is a very 

 powerful agent in modifying the characters of butterflies, 

 is well shown by the case of Polyoniinatiis phlccas. The 

 two broods of this insect are alike in Germany, while in 

 Italy the summer brood has the wings dusky instead of 

 copper-coloured. The period of development is exactly 

 the same in both countries, so that the change must, it is 

 argued, be attributed to the higher temperature of the 

 Italian summer. It has been noticed that in Italy a 

 large number of species of butterflies are thus seasonally 

 dimorphic which are not so in Central and Northern 

 Europe. 



Dr. Weismann lays great stress on the varied effects 

 of temperature in modifying allied species or the two 

 sexes of the same species, from which he argues that the 

 essential cause of all these changes is to be found in 

 peculiarities of physical constitution, which cause different 

 species, varieties, or sexes to respond differently to the 

 same change of temperature ; and he thinks that many 

 sexual differences can be traced to this cause alone with- 

 out calling in the aid of sexual selection. The general 

 result arrived at by the laborious investigation of these 

 phenomena is, that — " a species is only caused to change 

 through the influence of changing external conditions of 

 life, this change being in a fixed direction which entirely 

 depends on the physical nature of the varying organism, 

 and is different in different species, or even in the two 

 sexes of the same species;" and he adds : — "According 

 to my view, transmutation by purely internal causes is not 

 to be entertained. If we could absolutely suspend the 

 changes of the external conditions of hfe, existing species 

 would remain stationary. The action of external inciting 

 causes, in the widest sense of the word, is alone able to 

 produce modifications ; and even the never-failing ' indi- 

 vidual variations," together with the inherited dissimilarity 



