32 



SCIENCE. 



A DARWINIAN STUDY. 

 By Alfred R. Wallace. 

 For the benefit of those unacquainted with entomology 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 trans- 

 formations 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 peiceptible differ- 

 ence ; but in others there is a constant vaiiation, and some- 

 times this is so great that the two farms 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 

 color, 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 1830) no one doubted their being 

 altogether distinct. 



In order to learn something of the origin and nature of 

 this curious phenomenon Dr. Weisman has for many years 

 carried on a variety of experiments, breeding the species 

 in large numbers and subjecting the pupae to artificial 

 heat or cold for the purpose of hastening or retarding the 

 transformation. The result of these experiments 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 conditions that has yet been 

 tried has 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 hypo- 

 thesis 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 becom- 

 ing 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 to the ancestral type ; while the produc- 

 tion of the summer form {A. prorsa) from autumnal larvae is 

 impossible, because that form is the result of gradual de- 

 velopment ; and processes of development which have 

 taken thousands ofyears to bringabout cannot be artificially 

 reproduced in a single season. 



This hypothesis was supported by experiments with 

 another two-brooded species, Pieris napi, with similar re- 

 sults, the winter form being produced with certainty by the 

 application of cold to summer pupae ; and Mr. Edwards, in 

 America, has made similar experiments with the various 

 forms of Papilii a/a.r, finding that the summer broods can 

 he changed into the winter form by the application 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 Pieris uapi, which very closely resembles the 

 winter form of the rest of Europe, and this could never be 

 the least changed by rapidly developing the pupae under 

 the influence of heat. 



Another curious case is that of one of the Lycaenidac 

 (Plebeiusagestis) which exhibits three forms, which may be 

 designated as A, B, and C. The first two, 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 tin- pupa stage is a very power- 

 ful agent in modifying the characters of butterflies, is well 

 shown by the case ol Polyommatus phlaas. The two broods 

 of this insect are alike in Germany, while in Italy the sum- 

 mei l<rood has the wings dusky instead of copper-colored. 

 The period of development is exactly the same in both 

 countries, so that the change must, it is argued, be attribut- 



ed 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 temp- 

 erature ; and he thinks that many sexual differences can be 

 traced to this cause alone without calling in the aid of sex- 

 ual selection. The general result arrived at by the labor- 

 ious investigation ot 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 intern- 

 al causes is not to be entertained. If we could absolutely 

 suspend the changes of the external conditions of life, 

 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 ' individual variations,' together with the inherited 

 dissimilarity of constitution, appear to me to depend upon 

 unlike external influences, the inherited constitution itself 

 being dissimilar, because the individuals have been at all 

 times exposed to somewhat varying external influences." 

 The present writer has arrived at almost exactly similar con- 

 clusions to these, from a study of the geographical distri- 

 bution and specific variation of animal forms, as stated in 

 an article on " The Origin of Species and Genera," which 

 appeared in the Nineteenth Century of January last, and it 

 is gratifying to find them supported by the results of a very 

 different line of inquiry, and by the authority of so eminent 

 and original an observer as Dr. Weismann. 



A FOURTH STATE OF MATTER 1 



In introducing the discussion on Mr. Spottiswoode and 

 Mr. Moulton's paper on the "Sensitive State of Vacuum 

 Discharges," at the meeting of the Royal Society on April 

 15, Dr. De La Rue, who occupied the chair, good-naturedly 

 challenged me to substantiate my statement that there is 

 such a thing as a fourth or ultra-gaseous state of matter. 



I had no time then to enter fully into the subject ; nor was 

 I prepared, on the spur of the moment, to marshal all the 

 facts and reasons which have led me to this conclusion. 

 But as I find that many other scientific men besides Dr. De 

 La Rue are in doubt as to whether matter has been shown 

 to exist in a state beyond that of gas, I will now endeavor to 

 substantiate my position. 



I will commence by explaining what seems to me to be 

 the constitution of matter in its three states of solid, liquid, 

 and gas. 



I. First as to Solids : — These are composed of discon- 

 tinuous molecules, separated from each other by a space 

 which is relatively large — possibly enormous — in compari- 

 son with the diameter of the central nucleus we call molecule. 

 These molecules, themselves built up ofato/ns, are governed 

 by certain forces. Two of these forces I will hererefei to — 

 attraction and motion. Attraction when exerted at sensible 

 distances is known as gravitation^ but when the distances 

 are molecular it is called adhesion and cohesion. Attraction 

 appears to be independent of absolute temperature ; it in- 

 creases as the distance between the molecules diminishes; 

 and wire there no other counteracting force the result would 

 be a mass of molecules in actual contact, witli no moleculai 

 movement whatever — a state of things beyond our concep- 

 tion — a state, too, which would probably result in ihe crea- 

 tion of something that, according to our present views 

 would not be ///otter. 



This force of cohesion is counterbalanced by the move- 

 ments of the individual molecules themselves, movements 



1 "(in a Fourth State <>f Matter," in a letter to the Secretary of the 

 Royal Society. By \V. Crookcs, F.R.S. 



