584 



NA TURE 



{April 24, 1890 



which has hitherto been published only in various Journals 

 and Transactions of Societies. The book commences with 

 a chapter on ancient traditions, giving a chronological 

 table of the more important shocks which have occurred 

 since 79 A.D. The second chapter briefly discusses the 

 connection between earthquakes and volcanoes, a subject 

 of which we have apparently a good deal still to learn. 

 Then follow descriptions and illustrations of various 

 seismometers and seismographs, including the latest forms 

 devised by Profs. Gray and Milne. In this chapter 

 there are given several interesting comparisons of earth- 

 quake curves automatically recorded by the instruments, 

 and curves artificially produced by the application of 

 forces of known direction and magnitude. The pro- 

 pagation of shocks through land and water, and their 

 destructive effects, are also considered, the latter being 

 illustrated by sketches of some of the more remarkable 

 fractures and displacements which have been observed. 

 The last chapter summarises the suggestions which have 

 been made as to possible connections between earth- 

 quakes and astronomical and meteorological phenomena. 

 In conclusion, M. Girard points out the necessity for 

 continued systematic observations, and enumerates the 

 chief points on which further information is required. 



To those who know little or nothing of the subject, 

 M. Girard's little book will form an admirable intro- 

 duction ; and to the initiated it will be a handy book of 

 reference to its latest developments. 



La Photographic a la Lumiere du Magnesium. By Dr. 

 J. M. Eder. Translated by Henry Gauthier-Villars. 

 (Paris : Gauthier-Villars and Son, 1890.) 



This is a translation of a very interesting Httle German 

 work on the employment of magnesium light for the 

 purposes of photography, and will form a useful addition 

 to our photographic literature. The author first gives a 

 brief account of the earlier stages of the subject, taking us 

 back to the time when Bunsen and Roscoe, in the year 

 1859, indicated the considerable advantages the light of 

 magnesium presented for photo-chemical studies and 

 lighting. He then shows how Crookes afterwards 

 employed the light for photographic purposes. 



Amongst the. very first attempts of artificial lighting, the 

 wire of magnesium was used. It was burnt in a specially- 

 made lamp, and the light thus produced answered fairly 

 well for interiors, but was useless for portrait work, being 

 too harsh. The next advance was the employment of a 

 mixture consisting of the powder of magnesium, chlorate 

 of potassium, and a sulphide of antimony ; the light was 

 produced by igniting the mixture, which flared up instan- 

 taneously. The chief drawback to this method was the 

 great precaution that had to be taken during the mixing, 

 as the slightest blow caused an explosion. Saltpetre in 

 place of potassium was sometimes used so as to lessen 

 the chances of explosion. 



The methods described in chapters v. and vi. were 

 those which gave the best results. They consisted in 

 blowing powdered magnesium through a tube and allow- 

 ing this powder to come out at the other extremity into a 

 gas or candle flame ; the light thus produced was ex- 

 tremely actinic, and did not present any danger. The 

 lamps of Schirm and Loehr, illustrations of which are 

 given in these chapters, were on this principle, and gave 

 great satisfaction for portraiture, being worked by 

 means of a pneumatic india-rubber ball. Chapter vii. 

 treats of the combustion of magnesium in Oxygen, and in 

 it is described Piffard's apparatus for the production of 

 this light, which was found to be enormously increased 

 by the presence of the oxygen. The remaining chapters 

 deal with methods of taking groups by this artificial light ; 

 and there is a very interesting illustration of the pupil of 

 the human eye, photographed in a dark room by means 

 of the flash light, the exposure of which was so short that 

 the pupil had no time to contract. The book concludes 



with some hints on the precaution necessary to insure 

 successful development of the negatives taken by these 

 processes, and with a short appendix by M. Alexandre. 



LETTERS TO THE EDITOR. 



[ Tht Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents . Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.} 



Panmixia. 



But for his statement that I "cannot be sincere," I should 

 not have deemed it necessary again to answer Prof. Lankester ; 

 anyone who is read in the literature of Darwinism must already 

 have perceived that a further reply on my part is needless. An 

 accusation of insincerity, however, ought not to pass unnoticed ; 

 and therefore I will ask your more general readers to observe 

 the ground on which it has been made. 



In my answer to his original criticism I endeavoured to show 

 that Prof. Lankester " fails to distinguish between the cessation 

 and the reversal of selection," or, more particularly, between 

 panmixia and the economy of growth ; and this is the point 

 with regard to which insincerity is charged. Yet this is just 

 the point — and the only point — in dispute. I have always 

 represented that the cessation of selection is per se a cause of 

 degeneration, whether or not it be associated with the economy 

 of growth. Prof. Lankester, on the other hand, represented 

 that the cessation of selection is not per se a cause of degenera- 

 tion ; but merely a "state," which is precedent to, and contem- 

 poraneous with, the economy of growth — the latter being the 

 cause, while the former is but a condition to the occurrence of 

 this cause. Such, at any rate, appeared to me the only meaning 

 that could be gathered from his paragraph at the top of p. 488 ; 

 and it is now over and over again repeated in his last letter. 

 For instance : — " Cessation of selection must be supplemented by 

 economy of growth in order to produce the results attributed to 

 'panmixia.' And inasmuch as economy of growth as a cause 

 of degeneration involves the condition of cessation of selection, 

 Mr. Darwin in recognizing the one recognized the other. . . . 

 It is true that Mr. Darwin did not recognize that such unre- 

 stricted variation must lead to a diminution in size of the varying 

 ■psiTt without the operation of the principle of ^ economy of growth J 

 This was no strange oversight : he would have been in error had 

 he done so. . . . The term [' panmixia '], like its correlative 

 'cessation of selection,' does not indicate a principle, but a 

 natural condition: it does not involve the inference that a 

 dwindling in the size of the organ must result from inter-breed- 

 ing ; but simply points to a precidcnt condition" (p. 559: 

 italics mine).-^ 



Where, then, is the insincerity in saying that Prof. Lankester 

 does not perceive the distinction between the cessation of selec- 

 tion and the economy of growth as two totally different causal 

 " principles " ? Or what remains for me but to repeat, with all 

 sincerity, " he confounds the 'idea' of panmixia with that of the 

 economy of growth," and "fails to perceive the ' essence of the 

 idea' in the all-important distinction between selection as with- 

 drawn and selection as reversed " ? 



It is true that at the close of his last letter Prof. Lankester 

 admits, " when we consider shape and structure, and not merely 

 size, it is clear that panmixia without economy of growth would 

 lead to a complete loss of that complex adjustment of parts 

 which many organs exhibit, and consequently to degeneration 

 without loss of bulk." But how was it possible to surmise from 

 his first letter that he had in his mind such reservations as to 

 "shape" and "structure?" Or, indeed, how is it possible to 

 reconcile such reservations with the passages above quoted from 

 his last letter, to the effect that the cessation of selection is " not 

 a principle at all," but merely " a condition which alone cannot 

 produce any important result " ? Are we to conclude that in 

 Prof Lankester's opinion neither "a complete loss of complex. 



' I may remark that the term "cessation of selection " is not the " cor- 

 relative," but the synonym of the term "panmixia." And I may further 

 remark that the term "reversal of selection " is not, as Prof Lankester 

 suppo.ses, the synonym of the term "economy of growth." Economy of 

 growth, where useless structures are concerned, may determine a reversal of 

 selection ; but the reversal of selection may also be determined by many 

 other causes and conditions, which gre equally potent- or even very much 

 more pote.nt — in this respect. 



