May 1 8, 1882] 



NATURE 



5: 



careful discussion of the real meaning of the transforma- 

 tion of the Axolotl into Amblystoma. Experiments are 

 detailed showing that the metamorphosis may be induced 

 with much constancy by obliging the Axolotls, at a proper 

 stage of growth, to leave the water, when they lose their 

 gills and undergo a number of other changes constituting 

 a veritable metamorphosis. Dr. Weismann states that 

 many zoologists have expressed an opinion (which was 

 formerly held by himself) that this change is to be con- 

 sidered to be a true advance in development from a 

 species which had hitherto remained in the larval stage, 

 but which, through the influence of certain changed con- 

 ditions, now advances, per sattum, to a higher stage. 

 This view he gives many excellent reasons for considering 

 to be quite erroneous ; holding that the facts are best 

 interDreted by supposing that the animal formerly under- 

 went metamorphosis, but that owing to changed condi- 

 tions it was unable to survive in the perfect state, and 

 therefore remained in the larval condition in which it 

 acquired the power of reproduction. 



The causes which led to this change are believed to be 

 a progressive drying up of the Mexican lakes (as long 

 since proved by Humboldt), and a consequent increased 

 aridity of the atmosphere inimical to land amphibia. The 

 axolotl, therefore, presents us with a case of degeneration ; 

 and its metamorphosis under changed conditions in con- 

 finement is not due to any advance in organisation, but 

 is really a reversion to a not very remote ancestral habit. 

 The whole of the facts at present ascertained with regard 

 to these animals and allied forms in their native habitats, 

 are shown to agree well with this view, which is quite in 

 harmony with the author's explanation of seasonal dimor- 

 phism in butterflies, given in Part I. of the same work 

 (see Nature, vol. xxii. p. 141), and is also more in 

 accordance with the true principles of evolution than the 

 alternative hypothesis. 



The second, and concluding essay, is entitled " On the 

 Mechanical Conception of Nature," and is chiefly occu- 

 pied with an inquiry into the true character of variation 

 as the chief factor in evolution, and into the comparative 

 importance of external conditions, and the constitution of 

 the organism in determining the particular direction of 

 the course of development ; the object being to show that 

 all takes place according to fixed laws without the inter- 

 ference of any " teleological principle,'' whether in the 

 form of a " phyletic vital force " or the interposition of 

 any "designing power." The writers who^e views on 

 these subjects are combated are Von Hartmann and Karl 

 Ernst von Baer, and, after an elaborate and often subtle 

 argument, Dr. Weismann concludes that the facts can all 

 be explained on " mechanical" principles, or, as we should 

 say, by the action of fixed laws. He is however careful 

 to add that ihis does not imply a materialistic view of 

 nature. " Those who defend mechanical development 

 will not be compelled to deny a teleological power, only 

 they would have, with Kant, to think of the latter in the 

 only way in which it can be conceived, viz. as a Final 

 Cause." And on the great question of the nature and 

 origin of consciousness he thus expresses himself : — " If 

 it is asked, however, how that which in ourselves and in 

 the remainder of the animal world is intellectual and 

 ferceptive, which thinks and wills, is ascribable to a 

 mechanical process of development — whether the deve- 



lopment of the mind can be conceived as resulting from 

 purely mechanical laws ? I answer unhesitatingly in the 

 affirmative with the pure materialist, although I do not 

 agree with him as to the manner in which he derives 

 these phenomena from matter, since thinking and exten- 

 sion are heterogeneous things, and one cannot be con- 

 sidered as a product of the other." And he intimates that 

 the fundamental notion of conscious matter may get us 

 out of the difficulty. However this may be, he maintains 

 that the theory of selection by no means leads — as is 

 always assumed — to the denial of a teleological Universal 

 Cause, and to materialism, but only to the belief that any 

 mode of interference by a directive teleological power, 

 other than by the appointment of the forces producing 

 the phenomena, is, to the naturalist, inadmissible. "The 

 final and main result of this essay will thus be found in 

 the attempted demonstration that the mechanical concep- 

 tion of Nature very well admits of being united with a 

 teleological conception of the Universe." 



The work, of which the translation has now been com- 

 pleted by Mr. Meldola, must be considered a very im- 

 portant contribution to the theory of Natural Selection, 

 since it applies that theory to explain in the minutest 

 detail how the more prominent characters of several dis- 

 tinct groups of animals, not obviously useful to them, 

 may have been developed under its action. Such are the 

 distinct markings often occurring in two annual broods of 

 butterflies termed " seasonal dimorphism," the origin of 

 the markings of caterpillars, and the unexpected pheno- 

 mena of the transformation of the Mexican axolotl; and 

 we are therefore led to conclude that an equally careful 

 and minute study of other cases of difficulty would 

 probably lead to an equally satisfactory explanation. 

 This argument is not, however, conclusive, because the 

 cases here chosen may not be really test cases ; and. 

 among the countless forms of nature, and especially 

 among the higher animals, there may well be characters 

 or organs the origin of which are due to other and alto- 

 gether unknown causes. To students of evolution, Dr. 

 YVeismann's volume will be both instructive and interest- 

 ing, but it is a work that requires not merely reading, but 

 study, since its copious facts and elaborate chains t.( 

 argument are not to be mastered by a hasty perusal. 

 The book is beautifully got up and illustrated by a number 

 of coloured plates admirably executed in chromo-litho- 

 graihy, and it will form a handsome as well as an indis,-- 

 pensable addition to every naturalist's library. 



RED R. Wallace 



OUR BOOK SHELF 



Land and Freshwater Mollusca of India. Edited bv 

 Lieut-Col. H. H. Godwin-Austen, F.R.S., &c. Part I. 

 February, 1SS2. (London: Ta\lor and Francis.) 



This work is announced as " supplementary to Messrs. 

 Theobald and Hanley's ' Conchologia Indica,' " but it is 

 much more than a supplement, and is framed on far more 

 scientific principles. The "Conchologia Indica" was 

 published in 1S70; and 1 he editors in their preface say 

 that "after an interval of two or three years it is hoped 

 that materials for a supplement (the malacological portion 

 of which will be edited by Major Godwin-Austen) will be 

 accumulated." The " Conchologia Indica," however, 



