Mar. 14, 1872] 



NA TURE 



381 



unpleasant consequences of a late supper might ha.ve led 

 Mr. Thomson one step farther, and suggested to him the 

 probable habitat of the spirit when embodied. How brim- 

 ful of meaning to Mr. Thomson, then, must b 3 Shakespeare's 

 well-known utterance — " We arc such stuff as dreams 

 are made of" The particular merit which he claims for 

 himself as a discoverer is, that he lias realised to himself 

 this spirit-world " predicted of old to be in existence," 

 become conscious of himself as a " spirit in the world of 

 spirits," clearly distinct, " in rounded belief," as he puts it, 

 from that other entity, the body ; and he declares that 

 any one may make this awful discovery for himself if 

 he only has " faith," shuts himself off from the outer 

 world, and ponders long enough and with sufficient in- 

 tensity. If our author is really in earnest — and we can- 

 not but think he is— in trsing to fathom the mystery of 

 life and of consciousness, we recommend him to approach 

 the subject unprejudicedly from the side of physiology ; 

 for so long as a psychologist concerns himself with the 

 phenomena of h's "inner consciousness" alone, and 

 neglects the facts of his " outer man," his work is less than 

 half done, and he is as likely to succeed in arriving at the 

 whole truth as Columbus would hav^ been in discovering 

 America, had he contented himself with studying charts 

 and staring longingly across the Atlantic for forty years. 



On the Elevation of Mountains by Lateral Pressure; its 

 Cause, and the Amount of it, luith a Speculation on 

 the Orii^in of Volcanic Action. By Rev. O. Fisher, 

 MA, F.G.S, &c. (From the Trans, of Cxmb. Phil. 

 Soc. Vol. xi. part iii ) 

 This paper is of considerable interest as bearing upon the 

 question of the internal condition of the earth. Mr. 

 Fisher is of opiaion that the elevation of mountain chiins 

 and the phenomena of volcanoes can both be accounted 

 for on the hypothesis that the eartli is solid. He conceives 

 that "if a sufficient loss of heat has happened since the 

 stratified rocks were formed, to cause a slight diminution 

 in the volume of the earth, then the outer layer will have 

 become too large, and will have had to accommodate it- 

 self to the reduced spheroid ; and the lateral pressure 

 caused by the resulting failure of support will have given 

 rise to those foldings which have produced mountain 

 ranges ; " and an attempt is made by the author to " esti- 

 mate the lateral pressure which would arise in the outer 

 strata of the earth under such circumstances." Referring 

 to the results obtained by Archdeacon Pratt in India, 

 which seem to show that the density of the earth's cruit 

 beneath mountain chains is less than in other places, the 

 author thinks this is only what might have been expected 

 upon the supposition that the elevation of these moun- 

 tains is due to lateral pressure ; for it is evident that the 

 strata would to some extent be supported by the lateral 

 pressure which upheaved them. Here then, he thinks, 

 may be the origin of volcanoes ; — Diminished vertical 

 pressure will enable the interior layers of the crust to 

 pass into a state of fusion, and, " if from an independent 

 cause a partial passage towards the surface is opened for 

 molten rock containing highly heated water, the fluid will 

 convey to a level where the resistance is less the pressure 

 existing at a lower depth, and the force necessary to com- 

 plete a passage to the surface may be furnished by the 

 pressure of the molten rock and by the steam contained 

 within it." But, although Mr. Fisher believes that the 

 elevation of mountain chains and the phenomena of 

 volcanoes are both of them the result of the same funda- 

 mental causes, yet, he thinks, it would certainly be a 

 mistake to regard elevation as the consequence of vol- 

 canic action. He does not see how subterraneous lakes 

 of molten matter can account for the elongated form which 

 trains of volcanoes like those of the Andes affect ; nor 

 how such lakes should have shifted about from one region 

 to another at different geological epochs. His theory, 

 however, offers an explanation of the elongated form 



assumed by chains of volcanoes — the shifting of volcanic 

 activity to different regions at successive periods— the 

 spasmodic character of volcanic action, and other vol- 

 canic phenomena. J. G. 



LETTERS TO THE EDITOR 



[ Tlie Editor does not hold Id'nself responsible for opinions expressed 

 by his corre:pon tents. A'o njtice is taken of anonymous 

 com mufiications. ] 



The Placental Classification of Mammals 



A KEMARK made by I'rof Allen Thomson on this subject in 

 a la'e number of Nature induces me again to draw attention to 

 some objections I offered to ilis placental classification in a re- 

 view of Prof RoUeston's "forms of Animal Life" (Nature, vol. 

 i., p. 81). If this system fails to satisfy so sound a critic and so 

 accoii'plished an anatomist as Dr. Thomson, there must be some 

 serious deficiencies in it. No doubt De Blainville did good 

 service in calling attention to the wide distinction of Marsupials 

 and of Monotremes from other mammals ; but his names, Onii- 

 tho.liipjiii and Didclpliia, are inappropriate, and even mislead- 

 ing, and the skeletil characters of these two groups furnish quite 

 as important, anl fir more available, means of diagnosis. 



It adm'ti of questio.i wheihsr the divisions of the higher 

 mimmals, according to th; sim; system, are the most 

 natural, even if th; plic;nti '.vere tha best or^in by which to 

 define them. Ic is trae, as Pr if. Hiitley observes, that 

 the singularities which ally the elephant with the Rodentia 

 have been a matter of common remark since the days of 

 Cuvier, but the placental cla^sificatioa requires us t3 find 

 still more singuUr ties b.'tween the e'ephant and the Ca ni- 

 vora. On the other hand the Camivo-a lead down by the s^als 

 to the true Ce'acea, a line of connection broken by the placental 

 arrangement ; which is equally opposed to the more dou'otful 

 analogy of the whales with the Rum nants. And the third order 

 with deciduous zonary placentation, the isolated genus Ilyrax, 

 whatever may be thought of its relations to Rodentia on the one 

 hand and to Ungulata on the other, his at least more likeness to 

 either than to elephants and cats. Again, the diffetent placenta- 

 ti'in of Edentata may be held only an additional proof of the 

 looseness of an order held together chielly by negative characters, 

 but if we break it up, shall we obtain a more natural or conveni- 

 ent arrangement by placing the sloths with the Rumin.ants, Mnnis 

 with Cetacea and Perissodactyla, and Oryctcropus with Primates? 



No doubt embryolo^ical ch.aracteri are justly regarded as the 

 most important for revealing true affinities between animals. But 

 the tenacity of hereditary transmission, which gives them this 

 value, does not appear to belong to placental structure. The 

 placenta is more a maternal than a fcetal organ, especially as to 

 its deciduate or non-deciduate character, and should rather 

 rank with organs like the mamma than with the yelk-sac and the 

 amnion. 



There are, moreover, many practical objections to the placental 

 classification. The opportunities of obtaining knowledge on the 

 subject are few, the investigation is not always easy, and it can- 

 not be readily verified by subsequent observers. 



But the most important objection to De Blainville's system 

 is, that the perishable nature of the structures on which it is 

 based renders it impossible to apply the criterion to fossil animals. 

 It will probably be long before we shall have any notion of what 

 a Sirenian placenta is like ; it is only lately that we have learnt 

 what is the real placentation of so common a creature as the 

 rat, but we shall certainly never have the remotest idea of that 

 of a megatherium, a zeuglodon, or a rhytina. So that if it be 

 admitted — and surely no one will deny — that any classification of 

 animals which is to be more than a mere aid to the memory, must 

 include all known forms, recent or fossil, it follows that neither 

 placenta, nor brain, nor any other soft part, can l)e of more than 

 subordinate value in classification. On the other hand, it may 

 be fairly maintained that there is no group of mammals, and 

 scarcely one of the other Vertebrata, of undisputed importance, 

 which cannot be completely defined by the characters of the 

 skeleton. 



It is, I venture to think, rather the authority of such illustrious 

 names as Gegenbaur and Huxley than its own merits which have 

 recommended the placental classification of mammals. If we 

 regard the object of classification to be the setting forth of true 

 genetic relationships, all characters must be included, and among 



