July i6, 1896] 



NA TURE 



245 



OUR BOOK SHELF. 



A Concise Handbook of British Birds. By H. Kirke 

 Swann. Pp. 210. (London: John Wheldon and Co., 

 1896.) 



This is a handy and serviceable reference book on 

 British birds. It includes descriptions of the character- 

 istics, distribution, and habits of every species on the 

 British hst, and the information, though brief, is generally 

 sufficient for identification. The classification and 

 nomenclature followed is practically that of the British 

 Ornithologists' Union. The specific names of first 

 describers are, however, adopted, and sub-species or 

 races are distinguished by sub-numbers and trinominals. 

 Ornithologists, and bird-lovers generally, will find Mr. 

 Swann's book of practical value in the field, and very 

 useful for ready reference in the study. 



Practical Radiography. By H. Snowden Ward ; with 

 Chapters bv E. A. Robins and A. E. Livermore. Pp. 

 80. (The Photogram, Ltd., 1896 ) 

 There may be persons who furnish themselves with an 

 outfit for Rontgen photography without having a know- 

 ledge of either electricity or photography. For such 

 individuals, possessing aspirations without education in 

 physical principles, this book has been written. The 

 history of kathode rays and Rontgen's discovery occupies 

 seven pages of the book. There is a chapter on the 

 manufacture of an accumulator, and another describing 

 how to make an induction coil. The remaining five 

 chapters are taken up with descriptions of the apparatus 

 and methods of Rontgen photography. 



LETTERS TO THE EDITOR. 



\ The 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, 7-ejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.'] 



Are Specific Characters useful ? 



With the above title Mr. Alfred Kussel Wallace brought 

 before the Linnean Society on June iSan important communica- 

 lion, which derived additional interest from the fact that he 

 himself was present in full health and vigour, as well as from the 

 presence of a large number of naturalists who have given 

 allention to the questions arising from the consideration of the 

 theory of the origin of species by natural selection. 



In the course of the remarks which were offered by his 

 audience at the conclusion of Mr. Wallace's paper, I ventured to 

 ix>int out that the consideration of the class of phenomena which 

 .Mr. Darwin had described under the title "correlation of 

 variation," seemed to me to lead necessarily to the conclusion 

 that very often characters which are obvious and distinctive 

 marks of species may be not useful but useless, since such 

 obvious species marks may be only superficial and non-significant 

 phenomena "correlated" (as Mr. Darwin used that term) with 

 other less obvious but really important life-saving peculiarities, 

 which might quite well escape the observation of the desciiber 

 of "specific characters." -As instances of the phenomenon of 

 " correlation," I referred to those cited by Mr. Darwin, such as 

 the concomitance of a development of feathers on the feet with 

 the webbing of the toes in certain breeds of pigeons, the con- 

 comitance of abnormal dentition with hairlessncss of the body- 

 surface in Chinese dogs, the concomitance of deafness with blue 

 eyes in male white cats. A case which seemed to me most 

 striking and suggestive in connection with the utility of specific 

 characters was cited by me. It was that which had led Wells to 

 jiropound a d<x:trine of " natural selection " many years before 

 Darwin and Wallace had placed their views in 1858 before the 

 Linnean Society — a case which Mr. Darwin cited in later editions 

 of the "Origin of Species," and is familiar enough. Wells 

 pointed out in a memoir communicated to the Royal Society in 

 1813, that persons with dark pigment in the skin are relatively 

 immune to tropical fevers, as compared with fair.complexioned 



NO. 1394, VOL. 54] 



indi\iduals. He argued that owing to this property of dark- 

 skinned varieties of men, there would be a survival selection in 

 tropical regions of such varieties, and that probably, or at any 

 rate possibly, in this manner the black colour of tropical 

 races might be accounted for. I mention this more or less 

 hypothetical case as showing that an obvious and striking 

 character, namely, that of black pigment in the skin, might 

 become predominant, and conceivably might become a 

 "specific character," although the blackness was not in itself 

 a " useful," that is, a "life-preserving or progeny-ensuring'" 

 character, but merely the accompaniment of a power of resisting 

 malarial germs, which we now have reason to believe consists in 

 a special chemical activity of the leucocytes (phagocytes) of the 

 blood and other tissues. From the consideration of this and 

 other similar cases, I argued that many " specific characters " 

 (that is to say, as defined by Mr. Wallace, characters which 

 individually or in definite association with other characters con- 

 stantly occur in one species and not in the other species of a 

 genus) must be devoid of utility themselves, and appear merely 

 as the "correlatives" or " concomitants" of really effective life- 

 preserving or progeny-ensuring characters. I insisted, finally, 

 on the very great importance of the correlation of parts in 

 animal organisms, and the necessity of regarding animals (and 

 presumably also plants) as most highly-wrought mechanisms in 

 which no part can vary without the accompaniment of variation 

 in some remote and (in our present state of knowledge) un- 

 expectedly correlated part, and to a degree often excessive and 

 (in our present state of knowledge) un.iccountable. Thus, as 

 Mr. Darwin himself pointed out, the selection of a given favour, 

 able variation may lead to excessive variation in a remote region 

 of the organism, which in its turn will very often (but not neces- 

 sarily always or at once) become the subject of further selection. 

 Mr. Darwin appears to have deprecated, in conversation with 

 Mr. Thiselton-Dyer (according to the latters interesting state- 

 ment in the debate on Mr. Wallace's paper), the invocation 

 of this theory of "correlation" as an explanation of cases 

 of apparently useless parts in animals or plants when under 

 investigation, holding that our ignorance of the modes in which 

 parts may be serviceable to an organism is so great that we 

 should rather experiment and observe as to their possible utility 

 than advance a theory which dismisses further inquiry. Whilst 

 agreeing with Mr. Thiselton-Dyer as to the " immorality '' (as 

 he termed it) of a naturalist who favours theories which paralyse 

 his activity as an observer and experimentalist (on which subject 

 see the last paragraph of this letter), I yet think that, as 

 seekers after true knowledge, we are bound to face the complex 

 problem in all its aspects. The obvious character, as well as 

 many less obvious characters, which we note as distinguishing 

 one species from another, are not improbably, it must be 

 admitted, in many cases concomitant phenomena of some other 

 phenomenon which alone among them is effective in determining 

 the preservation of the life, or the production of progeny in the 

 case of the individuals so characterised. 



At the same time I think that it may well be maintained 

 that such secondary or concomitant characters are not long 

 allowed to remain non-significant, and that sooner or later they 

 fall under the moulding action of natural selection, becoming 

 as they increase in volume either useful or injurious. 



My chief object in writing this letter is to draw attention ti> 

 the views of Prof Weldon, who has for some time, as all 

 zoologists know, been occupied in tabulating a very large series 

 of measurements of growing crabs. When I had stated ni)- 

 views as to the importance of "correlation of variation," with 

 which Mr. Meldola and Mr. Wallace subsequently expressed 

 their complete agreement, Prof. Weldon declared, with some 

 expressions of reluctance and regret — due, as he was good 

 enough to say, from an old pupil to the teacher whom he is 

 about to denounce and demolish — that to attempt to say which 

 of two or more correlated growths is the cause of survival is 

 unreasonable, and that when I suggested, even as a matter for 

 consideration, that a certain germ-slaying quality in phagocytes 

 accompanying a pigmented skin, rather than the pigment itself 

 in the skin, is the cause of the survival of dark-skinned people 

 in malarial regions, I was "absolutely illogical." " It is," said 

 Prof Weldon, " impossible logically to separate these two cor- 

 related phenomena. The coloured skin is as much a cause of 

 the survival of the dark man as is the germ-destroying property 

 of his blood." 



I was at the time entirely unable to appreciate the drift 

 of Prof. Weldon's thought. I was not prepared for an empty 



