/it UC 12, 1884] 



NA TURE 



163 



where k = length a b, will be the equation of R. lie mentioned 

 also that Prof. Willis had proved that nil working teeth on cor- 

 responding pitch-curves are roulettes of the same curve and 

 generated by the same point, and also that, with circular pitch- 

 curves, teeth which were involutes of circles concentric re- 

 spectively with the corresponding pitch-curves, and having the 

 same centre of similitude with them, would work correctly, and 

 stated that these theorems taken together had suggested to him the 

 question, What curve rolling on a circle will generate, and by what 

 point, the involute of a concentric circle ? Thi - curve he proved 

 to be an equilateral spiral whose pole is the generating point. — 

 On the alleged effect of magnetism on the human body, by Prof. 

 W. F. Barrett. In a recent address SirW. Thomsondrew attention 

 to the "marvellous fact " that a powerful magnetic field appeared 

 to exert no action on the human body, and stated his conviction 

 that, "if there is not a distinct magnetic sense, it is a very great 

 wonder that there is not." The object of the present paper was 

 to describe certain facts which had come under the author's 

 observation, and which pointed in the direction of a distinct 

 sensory and therapeutic effect produced by a powerful electro- 

 magnet upon certain individuals. A careful examination of 

 upwards of 100 persons had led to the discovery of three indi- 

 vidual who could instantly detect by their sensations when the 

 current was put on or taken off a large electromagnet, between 

 the poles of which their heads had been placed. In an absolutely 

 darkened room a singular luminous glare was also ver the 



magnetic poles by these three observers. Every care was taken 

 to avoid collusion or chance coincidence, and the observers had 

 no means of knowing by any other means when the current was 

 put "on" or "off." If a distinct magnetic sense exist, as these 

 experiments seem to suggest, it is doubtless rare and fitful, 

 depending possibly on the state of the percipient's health. The 

 author then described experiments that had been made by 

 Charcot in Paris, Dr. W. H. Stone at St. Thomas's Hospital, 

 London, and Prof. Dresehfeld at the Manchester Infirmary, to 

 ascertain the pathological effect of a powerful magnet. The two 

 former authorities had noticed the transference of sensation pro- 

 duced by magnetism in patients suffering from hysteria or hemi- 

 anesthesia, under conditions which appeared to preclude the 

 possibility of imagination coming into play. Dr. Dresehfeld 

 describes three cases which came under his own observation 

 where anaesthesia was cured by a large electro-magnet. In one 

 case, particulars of which were published in the British Medical 

 Journal, every care was taken to eliminate causes other than 

 the specific effect of magnetism, and there seemed no doubt that 

 the patient's complete restoration to health was due to the latter 

 cause alone. In conclusion Prof. Barrett remarked that should 

 the therapeutic value of magnetism in certain specific disorders be 

 established, it would obviously give no support to certain mag- 

 netic appliances which are sold as nostrums for all diseases, and of 

 the specific value of which he was not aware that there exists the 

 smallest medical evidence. — On the substitution of sodium bichro- 

 mate for the potassium salt in bichromate batteries, by Prof. W. 

 F. Barrett. Prof. Barrett stated that a week or two ago Mr. 

 Moss placed in his hands a specimen of bichromate of soda, and 

 asked him to try whether it would efficiently replace the potash 

 salt which is invariably employed in bichromate cells. The 

 result of his examination showed that there was no appreciable 

 difference between the electromotive force, the internal resistance, 

 and the constancy of the two cells, charged with equal weights 

 of the soda and of the potash salt respectively. — Reply to the 

 criticisms of M. Lcewy, by Howard Grubb, F.R.S. (see Nature 

 for May 29, p. 100). 



Section of Natural Science. — V. Ball, F.R.S., in the chair. — 

 On the origin of freshwater faunae, a study in evolution, by Prof. 

 W. J. Sollas, D.Sc, F.G.S. The poverty of freshwater faunae 

 as compared with marine is commonly attributed to a supposed 

 inadaptability on the part of marine organisms to existence in 

 fresh water. That this explanation is inadequate is shown by the 

 existence of freshwater jelly-fish such as Umnocodium, and still 

 more directly by the experiments of Beudant, who succeeded in 

 accustoming several kinds of marine mollusca to a freshwater 

 habitat. The view of Von Martens that the severity of a fresh- 

 water climate is prohibitive of the existence of most marine forms 

 in rivers is insufficient, and a more thoroughgoing explanation is 

 necessary. This is to be found in a study of the means by which 

 the distribution of marine animals is secured. In the case of 

 stationary forms free-swimming embryos are distributed over 

 wide areas by currents, and they can never pass fn iin the sea into 

 rivers, in which the current is always directed seawards. Nor, 



probably, could an attached form once introduced into a river 

 permanently establish itself so long as its propagation took place 

 exclusively through free-swimming larvae, for these would gradu- 

 ally be borne out to sea. Hence, freshwater animals should not, 

 as a rule, pass through a free larval stage of existence, nor, as a 

 matter of fact, do they. In Hydra, freshwater sponges, and 

 Polyzoa, the young usually emerge from a horny cyst in the 

 complete state. In the Unionidae, the glochidium s age provides 

 for distribution without involving a seaward journey. The young 

 of freshwater mollusks do not enter upon a free existence till 

 they are similar to their parents, and Paludina is viviparous. The 

 suppression of a free-swimming larval stage not only occurs in 

 freshwater but in many marine invertebrates. This is connected 

 with the fact that the larval stage is in a position of disadvantage 

 as compared with the adult. Hence there is an advantage to the 

 organism if the larval stage can be passed overin astate of seclusion. 

 From this various other modifications follow ; development in 

 seclusion involves a supply of accessible food, hence the appear- 

 ance of yelk and other kinds of nourishment furnished by the 

 parent to the imprisoned embryo. Again, the secluded larva being 

 spared the drudgery of working for its own existence, and sup- 

 plied with nutriment in a form that puts the least tax on its 

 digestive powers, a larger balance of energy remains available 

 for metamorphic changes. Thus arise the phenomena of accele- 

 rated and abbreviated development. Further, the shortening of 

 the larval life probably leads to the lengthening of the adult life, 

 and shifts the chances of variation and selection forward into the 

 adult stage. Thus animals which hatch out in a complete state 

 will most probably suffer modifications of that state, and not of 

 previous ones, except very indirectly. Here we discover a direct 

 tendency towards a mode of development which explains the 

 " arborescent " character of our zoological classifications, i.e. the 

 tendency of the tree of life is now to produce leaves rather 

 than new branches. In the case of freshwater fauna very direct 

 reasons have existed for the suppression of the free larval stage. 

 In this connection may be noticed the richness in species and the 

 poverty in genera of the freshwater mollusca. In discussing the 

 origin of freshwater fauna; there are three hypotheses from which 

 we have to select : (1) that marine forms have migrated into 

 rivers ; (2) that they have migrated into marshes and thence into 

 rivers ; and (3) that marine areas have been converted into fresh- 

 water ones. The last course has been the most usual, especially 

 in the case of non-locomotive forms. Hence the origin of fresh- 

 water invertebrates is connected with the great movements which 

 have affected the earth's crust. The earliest well-known lacus- 

 trine areas are those of the Old Red Sandstone, in one of which we 

 meet with the earliest known freshwater mollusk, Anodonta jitkesii 

 (Forbes). Thelakesof the Permo-Triassic period contributed addi- 

 tions to the freshwater fauna of the globe. The Neritid^ and Ceri- 

 thiada are probably post-Palaeozoic families, and, as the Neri- 

 tina and Melaniidce are so closely connected with them, they 

 may be regarded as their collateral or direct descendants, and 

 thus may have originated in Triassic lakes, but not earlier. 

 Other genera probably arose at the same time ; the occurrence 

 in Cretaceous deposits of Unio, Physa, Valvata, and Lymnea in 

 the Nearctic, Palaearctic, and Oriental regions, suggests a high 

 antiquity for these genera ; and they may have existed in Palaeo- 

 zoic times. The lakes of the Tertiary period furnished probably 

 further contributions to our freshwater fauna, such as Lilho- 

 glyphus and Dreissena. Thus, existing freshwater genera are 

 probably descended from marine forms which became meta- 

 morphosed in the waters of the Devonian, Triassic, and Tertiary 

 lakes. In the lakes of Central Africa the Tertiary freshwater 

 fauna still survives, nearly all of the genera from Lake Tan- 

 ganyika being referable to genera already in existence in Meso- 

 zoic and Tertiary times. The lakes of the Northern Hemisphere 

 received on subsiding beneath the glacial sea such Arctic forms . 

 as Mysis relicta and Pontopcreia nffinis, but most of their existing 

 inhabitants have re-entered them since their emergence from the 

 sea. 



Paris 



Academy of Sciences, May 26. — M. Rolland, president, in 

 the chair. — Observations of the small planets made with the great 

 meridian instrument of the Paris Observatory during the first 

 three months of the year 1S84 ; communicated by M. Mouchez. 

 The observations of January 11, February 11 and 12, March 14 

 and 15, were made by M. P. Puiseux, all the rest by M. H. 

 Renan. — Remarks on the sense of sight in its relations with 

 different colours placed in juxtaposition, by M. Chevreul. — 

 Fresh experiments made with a view to determine the locality 



