A7ig. 17, 1 8 76 J 



NATURE 



329 



W/ 



printed). Slight notices have also been given of spectrum- 

 photography, which we hope to find fuller in a new edi- 

 tion, and of the effect of colouring matters in modifying 

 the action of light on various reagents, the study of which 

 latter point appears to be undergoing considerable deve- 

 lopment both at home and abroad. 



Taken as a whole, the book is an admirable guide to 

 one who has some considerable knowledge of the subject, 

 or for giving the general points of the art to an ordinary 

 reader, but we do not think that it is equal to Van 

 Monckhoven's book as a practical treatise on the art. It 

 abounds, however, in valuable hints and suggestions, and 

 we would recommend Chapter XII., " On the Correctness 

 of Photographs," to the attention of everyone who wishes 

 to become a competent photographer, whether for the 

 purposes of Science or of Art. With regard to the latter 

 we cannot do better than conclude with the following 

 quotation from the chapter in question, page 129 : — 



" It may, perhaps, excite surprise that the writer 

 ascribes greater truth to painting than to photography, 

 which is generally regarded as the truest of all methods 

 of producing pictures. It must be self-evident that this 

 remark can be made only of the works of masters . . . 

 the picture of the photographer is not self-created. He 

 must test, weigh, consider, and remove the difficulties 

 which oppose the production of a true picture. If his 

 picture is to be true he must take care that the charac- 

 teristic is made prominent and the accessories subor- 

 dinate. ... To do this he must, of course, be able to 

 detect what is characteristic and what accessory in his 

 original. The sensitive plate of iodide of silver cannot 

 do this ; it receives the impression of all that it has 

 before it, according to unchangeable laws. . . . The 

 photographer will not, indeed, be able to control his 

 matter like the painter, for the disinclmation of models 

 and optical and chemical difficulties often frustrate his 

 best endeavours ; hence there must always be a difference 

 between photography and a work of art. This difference 

 may be briefly summed up by saying that photography 

 gives a more faithful picture of the form, and art a more 

 faithful picture of the character." 



R. J. Friswell 



OUR BOOK SHELF 



Lcs Insectes J Traite Elcinentaire (VEntomologie^ coinpre- 

 nant PHisioife des Especes utiles et de leurs Produiis, 

 des^ Especes nuisibles et des Moyens de les detruite, 

 P Etude des Metamorphoses et des Mceurs, les Precedes 

 de Chasse et de Conservation. Par Maurice Girard. 

 (Paris : Bailliereet Fils, 1873-76.) 



As a compilation this work evidences a considerable 

 amount of industry; judging, however, by the various 

 memoirs quoted in the first 240 pages, it would appear 

 that the author's researches have not extended to a 

 much later date than the year 1868, a fact which will 

 unquestionably detract very greatly from the value of his 

 generalisations. 



The author's object being to unite in one book the 

 classification, geographical zoology, and economy of 

 insects, he divides his introduction into the following 

 heads : — i. Anatomy and Physiology ; 2. Instinct and 

 Intelligence ; 3. Collecting and Preservation ; 4. Palae- 

 ontology ; 5. Geographical distribution ; 6. Species and 

 Classification ; the consideration of which subjects occu- 

 pies 229 pages. 



Owing to the bulk of the work (which, although up to 

 the present time it has only dealt with three orders of 

 insects, nevertheless extends to 840 pages), we cannot 

 strongly recommend it as a pocket companion ; still the 



student of entomology, particularly if he has a taste for 

 preserved viands warmed up, should certainly find a place 

 for it upon his library shelves. 



The plates are clearly defined and abound in instructive 

 details, the only drawback being that they are for the 

 most part reproductions of the illustrations to Guerin's 

 " Iconographie du R6gne animal j " it is, however, 

 satisfactory to note additional representations of an 

 anatomical character, as also of certain highly interest- 

 ing cave-inhabiting species. A. G. B. 



LETTERS TO THE EDITOR 



[ The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscritts. 

 No notice is taken of anonymous communications. "X 



Protective Mimicry 



In the last number of Nature Mr. Murphy brings forward 

 the following argument against natural selection, with reference 

 to protective mimicry. He advances two classes of cases in which 

 he urges the improbability of the occurrence of a first variation 

 in the requisite direction. " One of these is the change of colour 

 with the season of such animals as the ermine, which is brown in 

 summer and white in winter. Had the ermine been either 

 permanently brown or permanently white, there would have been 

 nothing wonderful in it, but it seems impossible that the cha- 

 racter of becoming white in the winter and brown in the summer 

 could ever have originated in ordinary spontaneous variation, 

 without a guiding intelligence." 



Now Pallas as quoted by my father ("Descent of Man," 

 second edition, p. 22^ and 542) states that wolves, horses, and 

 cattle, as well as nine other kinds of mammals become lighter 

 coloured during the winter ; and several well maiiied cases of a 

 similar change of tint in the winter coat of horses in England 

 have either been brought under my father's notice, or have been 

 observed by himself. It is impossible to suppose that these 

 instances of a similar change occurring in widely distinct animals 

 can be put down as partial reversions to an ancestral habit of 

 turning completely white during the winter months. We may 

 therefore presume that they are due to the "direct action of 

 the conditions of life ; " we might perhaps compare them to the 

 greyness accompanying the impaired nutrition of old age ; or to 

 that caused by injuries, e.g. in the hair about old sores on the 

 withers of horses ; or again to the extraordinary recorded case 

 of temporary greyness of the eyebrow accompanying frontal 

 neuralgia. 



But to whatever cause these slight changes of colour In certain 

 quadrupeds taking place at the beginning of winter are due, there 

 can be no question that they supply the identical "first varia- 

 tions," whose occurrence Mr. Murphy thinks so "infinitely im- 

 probable." It is impossible to doubt that with such material to 

 work on, a process of rigorous natural selection might develop 

 almost any degree of seasonal change of fur. "The roe, for 

 instance, has a red summer and a greyish white winter coat ; 

 and the latter may perhaps serve as a protection to the animal 

 whilst wandering through the leafless thickets, sprinkled with 

 snow and hoar-frost." If the roe "were gradually to extend its 

 ran^e into regions perpetually covered with snow, its pale 

 winter coat would probably be rendered through natural selec- 

 tion, whiter and whiter, until it' became as white as snow." 

 ("Descent of Man," p. 542). Mr. Murphy also adduces the 

 manner in which the chameleon, and certain fish, protect them- 

 selves by rapid changes of colour. He remarks that it seems 

 ' ' utterly impossible for such a character to originate in spon- 

 taneous variation." It would be taking up too much space to 

 enter into this subject ; it may, however, be worth noting that 

 according to Pouchet, under certain conditions the changes of 

 colour are only developed in the turbot in several days. Again 

 J. Bedriega asserts that various parts of the body in certain 

 lizards are permanently altered in tint by exposure to the sun ; 

 and he states that the mechanism by which this change is 

 effected is precisely the same in principle, as that to which the 

 variations in pigmentation are due in the chameleon. Briicke 

 and others have shown that in the chameleon the changes of tint 

 may be produced by agencies having no connection with protec- 

 tion ; for example, by the excitements of anger and sexual 

 passion, by illness, and by local Irritants and nerve-stimulation. 



