5o6 



NA TURE 



[March 31, 1898 



faculty of cell-regeneration, due to tlie diminution and 

 loss of the stimulus of functional excitation. In the case 

 of limbs, the functional excitation is supplied by the 

 stress to which the limbs are subjected in supporting the 

 body weight. The primitive limb consisted of many 

 rays ; eventually five only were directly used for sup- 

 port, the remaining rows degenerated, and are now 

 represented only by the " sesamoids." In further 

 development of races, groups arose in which a lesser 

 number of digits than five were used for support : the 

 digits which were no longer used lost the stimulus of 

 functional excitation and became degenerate, and their 

 degeneracy was represented in ontogeny by a retard- 

 ation in histological differentiation. Dr. Mehnert is 

 clearly a Neo-Lamarckian as well as a follower of Roux, 

 and he speaks, as clearly as Lamarck himself might 

 speak, of the importance of individual effort in increas- 

 ing or diminishing functional excitation. But there 

 is one thing that Dr. Mehnert does not succeed in 

 accounting for, nor has any Neo-Lamarckian yet suc- 

 ceeded in accounting for it, the constitution of the germ, 

 which is such that the pentadactyle hand is first formed, 

 even when it is destined to be monodactyle or didactyle 

 in the adult, and this in an embryo developing under 

 conditions which preclude the action of functional 

 excitation. His principle of diminished cell-regeneration 

 following upon diminished functional excitation obliges 

 him to reject the teachings of those authors who assert 

 a primary blastogenic phylogenesis, but he is constrained 

 to admit some sort of preformation in the germ cell, and 

 falls back on the specific energy of affinity possessed by 

 every atom according to its position in the periodic 

 system of elements. The course of speculation which 

 starts from such premises can hardly lead to useful con- 

 clusions. The following is an example of the author's 

 generalisations. 



" Ontogenetic evolutions are only the consequential 

 manifestations of phyletic epigenesis, which again is 

 in itself only a specialised evolution of the molecular 

 energies and affinities which integrate the individual. 

 That which in the earlier periods of the earth was 

 epigenesis, or, as one may now say phyletic evolution, is 

 now become ontogenetic evolution." 



One can see what he wants to explain, but one cannot 

 admit that he explains it. Towards the close of his 

 work he states that — 



" individual growth and development is a mosaic-work 

 of cells and organs produced by mass correlation which, 

 as a result of different phyletic functional efforts, charges 

 the germ with different regenerative energies." 



The same idea might be expressed much more simply. 

 The problem is, how and by what mechanism are varia- 

 tions in adult structures able to affect the germ in such 

 a manner that they may be reproduced in the next 

 generation ? Mehnert seeks to prove that they do affect 

 the germ, but he has not succeeded in suggesting the 

 manner in which they can do so. Weismann's con- 

 tention that acquired characters are not inherited has 

 yet to be shown to be untenable, and his position will 

 hardly be shaken by an argument which invokes the 

 individual efforts of ancestors with limbs of theoretical 

 construction in order to explain the observed facts in 

 the ontogenies of their presumed descendants. 

 NO. 1483, VOL. 57] 



RADIATION VISIBLE AND INVISIBLE. 

 Light Visible and Invisible. By Prof. Silvanus P. Thomp- 

 son, D.Sc, F.R.S., &c.. Principal of, and Professor 

 of Physics in, the City and Guilds Technical College, 

 Finsbury. Pp. xii -f- 294. (London : Macmillan and 

 Co., Ltd., 1897.) 



THIS is an age of rapid growth of scientific know- 

 ledge when the theory of to-day becomes the 

 established fact of to-morrow, and in no province have 

 our ideas shown a more rapid advance than on the 

 subject of radiations in the ether. First of all, the 

 electromagnetic theory of Maxwell upheld that light was 

 an electrical phenomenon, and this received its confirm- 

 ation by the experimental genius of Hertr, and the 

 subject of optics thus became attached to the domain of 

 electricity. Later came the discovery, by Rontgen, of 

 a kind of radiation entirely different from anything 

 before known, and this was soon after followed by a dis- 

 covery of a type of invisible radiation emitted by 

 uranium and its salts, which apparently possess proper- 

 ties intermediate between ultra-violet light and Rontgen 

 rays, but the cause of whose production is at present 

 one of the mysteries of science. Besides these, many- 

 other types of radiation, either apparent or real, have 

 been noted, and the subject of transformation of radia- 

 tions at the surface of bodies is now engaging the 

 attention of many observers. The last few years has 

 thus been an era of unexampled activity in the study 

 of radiations, and there is considerable evidence that 

 this activity will be productive of still further results in 

 the near future. 



In this little volume — " Light Visible and Invisible" — 

 Prof. Silvanus Thompson has published in full the si'x 

 lectures delivered at the Royal Institution at Christmas 

 1897. At the outset we feel that the title of these 

 lectures is rather a misnomer, for even a most imagin- 

 ative person would hardly have expected that the volume 

 was to include, under the title " Light Visible and In- 

 visible," a lecture on the subject of electromagnetic 

 waves, as well as a discussion of the properties and 

 production of Rontgen rays. 



These lectures are of necessity popular, and, as the 

 author has very well said in his preface, 



"two things are expected of a lecturer who undertakes 

 a course of Christmas lectures at the Royal Institution. 

 In the first place, his discourses must be illustrated to 

 the utmost extent by experiments. In the second, how- 

 ever simple the language in which scientific facts and 

 principles are described, every discourse must sound, at 

 least, some note of modernity, must reflect some wave 

 of recent progress in science " 



After reading the well-illustrated volume before us, no 

 one will be disposed to deny the author has fulfilled 

 the conditions laid down in the preface. The subject 

 is very simply treated, and abounds with experimental^ 

 illustrations ; and though there is little, if anything, in 

 the volume with which a scientific student would not be 

 more or less acquainted, we cannot but admire the at- 

 tractive way in which the information is laid before us. 

 It came rather as a surprise to us, however, to find the 

 " note of modernity " strongly sounded on the now well- 

 worn subject of Rontgen rays. 



The first lecture opens with a discussion of the 



