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NATURE 



[January 4, 1894 



EWART'S INVESTIGATIONS ON ELECTRIC 

 FISHES} 



THIS is a magnificent memoir containing the very in- 

 teresting observations of Prof. Ewart concerning 

 some of the most important chapters of comparative 

 anatomy. Everyone who has an idea of the enormous 

 difficulties connected with these investigations will ad- 

 mire the great skill and successful perseverance with 

 v> hich the author has followed up many dark problems 

 anc thrown light upon a number of the most obscure 

 questions. 



My studies have been in the same direction for nearly 

 twenty years, and I congratulate my companion in work 

 upon liis great success. 



It might appear a bold attempt for a foreigner to 

 debate the complicated problems treated in the work ; 

 but, on the other hand, there is apparently a strong in- 

 terest attached to the endeavour to enlarge the field of 

 international intercourse, and this will serve as an excuse 

 for any awkwardness of language. 



There is no doubt that science is the chapter of know- 

 ledge most entitled to international treatment, and 

 Prof. Ewart himself has done his best to acknowledge 

 the merits of foreign authors. Still, I wonder whether 

 he is aware of the fact that in many places 

 his deductions bear a more or less national character. 

 The proof of that fact cannot be compressed in a few 

 notes, but it may suffice to point out the places where 

 the differences of treating these matters between British 

 and continental writers seem to be most apparent. 



Everybody will probably agree that the whole of Prof. 

 Ewart's work deserves very high praise. The plates, which 

 have been accurately drawn by the author himself, are 

 beautifully printed, and yield very ample and useful 

 instruction to anyone who wants anatomical and histo- 

 logical information about these interesting, and yet very 

 imperfectly known, organs. They give a clear account of 

 the immense work the author had to accomplish before 

 he could give so exact and complete representations of the 

 electric organs as well as the cranial nerves and the 

 sense organs. 



It will remain to Ewart's undisputed credit that he has 

 brought before the public a large amount of information on 

 the anatomy and histology concerned. The explanation of 

 the plates facilitates the understanding of them, and forms 

 the connecting link between the figures and the tenourof 

 the deductions. It is proved by comparing a great number 

 of organs in different species of Raja, that there are two 

 distinct kinds of electric organs, viz. " cup-shaped," which 

 occurred in R. circularis, radiata and fiillonica of the 

 British seas, in R. eglanteria from abroad, and, on the 

 other hand, " disc-shaped," which he found in R. bails, 

 macrorhynchus, alba, oxychynchiis, tnaailata, clavata, 

 and microocellata. 



Everywhere Ewart confirms the statement of former 

 writers, that the electric organs were derived from muscles 

 of the tail which became changed into electric tissue. 

 With great care and skill he has followed the develop- 

 ment of these organs in the embryo, and showed how the 

 muscles gave up their firm hold on the sinews, and 

 shrivelled up to discs or cups. 



There remains yet one difficulty to be overcome, which 

 the author has not considered ; that is to say, he finds 

 the termination of the nerves for each element at the 

 proximal ^«^ of each muscular club, and compares them 

 (as other authors do) to the motor end plates of common 

 muscles, which are fixed alongside the striped fibres, 

 and not at the end. 



That is. of course, a subordinate question, and I am very 

 glad to repeat here what I have stated in former publica- 

 tions of mine, that the muscular origin of the electric 



1 " Electrical and Lateral Sense Organs, and on the Cranial Nerves ol 

 Elasmobranchs." By Prof. J. C. Ewart. (Edinbui-gh, 1893.) 



NO. 1262, VOL. 49] 



organ in the skate also appears beyond doubt, and that I 

 thoroughly agree with Prof. Ewart, not only in that 

 principal question, but also in his deductions regarding 

 the phylogenetic development. 



One of the general deductions appears rather strange, 

 not only to me, but to most authors on the continent 

 interested in the matter. How is it possible to consider 

 the electric organs of the skate as such of " vestigial char- 

 acter" before any evidence is given in favour of a 

 retrograde development, which takes place at any period 

 of life ? Are not all the statements of Ewart, as well as 

 of former authors, clear proofs that the development is 

 progressive, or at least resting at a certain degree of 

 perfection, after having left the starting-point (muscular 

 tissue) only for a comparatively short distance .'' 



The organs might still advance to further perfection 

 (which I presume they do), or they might become rudi- 

 mentary again ; but so long as there is only progressive 

 and no retrograde motion in the development, it is hardly 

 worth while to argue about the probability of their 

 vestigial character. 



I differ only so far from Ewart, as he does not convince 

 me that the electric organs of the skate are as equally 

 perfect as the organs of the Torpedo. In the skate, 

 and up to a certain degree also in Mormyrus, the striated 

 layer of the organ is histologically and optically (in pol- 

 arised light) proved to be the rest of the original muscular 

 tissue. If there is so much left of the former state of 

 things, it proves, in my opinion, that the process of 

 transformation going on is not so far perfected as in 

 another case (Torpedo), where nothing of the muscular 

 character is left. 



I may be permitted to quote here a suggestion I 

 made in a former number of this journal (January 19, 

 1893) regarding the probable way of phylogenetic de- 

 velopment in these organs. It seems to me possible 

 that a kind of physiological alteration changes certain 

 muscles so gradually into electric tissue that a compara- 

 tively still imperfect element under favourable circum- 

 stances might give an electric shock, which proves useful 

 to the skate for maiming small animals upon which it 

 preys ; and so the fish might continue to improve the 

 organ by using it. 



Ewart {R. circularis, p. 546) argues exactly in the 

 same way as I did, but hesitates to assume that weak 

 electric shocks might be of any use to the skate. It 

 should be kept in mind that small aquatic animals are 

 often extremely sensitive to electricity, and that an un- 

 expected weak electric shock startles even a human 

 individual. At another place, where the author treats 

 the same theory, and grants the possibility of all the 

 other presumptions, he holds back from the universally 

 accepted principle, that constant use makes an organ 

 increase. (Skate, p. 41 1). 



Of course, up to a certain degree, the phylogenetic de- 

 velopment of the electric organs contains still a good deal 

 of mystery, and will, I fear, always lack the scientific 

 proof so eagerly looked for ; but I must repeat my 

 conviction, that it is easier to imagine the trans- 

 formation of striped muscle in electric tissue, than 

 to explain by natural selection the development of any 

 distinct, lively-coloured pattern on the wing of an insect. 



The cautiousness of the author is, however, to be 

 praised, and it is very interesting to follow his argu- 

 ments about the pro et cofitra in these complicated 

 matters. 



I cannot admire as much another chapter of his paper, 

 where Prof. Ewart does not seem to be quite up to the 

 international mark ; it is that in which he compares the 

 number of electric elements in the different electric fishes. 

 (Skate, p. 397). The total of electric elements for each 

 organ in Torpedo marinorata he gives as 250,000, and in 

 spite of the great difference of this number with the sum 

 found by other writers before Ewart, he does not say one 



