September 28, igi 1] 



NATURE 



41, 



there is a carefully selected bibliography at the end of 

 c;ich chapter, and that the references are punctiliously 

 Accurate. 



The third part of the book gives a systematic 

 account of the whole phylum of vertebrates, and takes 

 due notice of the extinct forms. There are many in- 

 teresting detailed expressions of the author's judg- 

 ment, e.g. his treatment of the Ratitas as a hetero- 

 geneous group derivable from at least three stocks, or 

 his reuniting of Marsupials with Eutheria ; but the 

 outstanding feature in this section is to be found in 

 fie numerous carefully drawn up schemata showing" 

 distribution in time and probable affinities. There arc 

 twenty of these, condensing" much reflection. 



In the concluding section of his book, Prof. Vialleton 

 deals analytically with the problem of the evolution 

 of vertebrates. He discusses the origin of organs, 

 and makes much of Kleinenberg's theory of substitu- 

 tion ; he distinguishes between well-established genetic 

 pries and morphological series (so often mixed up 

 together, e.g. in connection with the evolution of 

 Equidas); he recognises the importance of paying" 

 mon- attention to the phenomena of convergence ; he 

 gives an admirable discussion of correlation and of 

 vestigial org-ans. Passing to the actual data bearing 

 on the phylogeny of vertebrates, he marshals the 

 pakcontological facts in a masterly way, and discusses 

 such ]m >ints as the successive appearance of classes, 

 the occurrence of generalised types and transitional 

 Bpes, the absence of the latter at phyletic bifurcations, 

 tli' extinction of types, and the indubitable progress 

 from age to age. Turning to embryological data we 

 find an admirable critical discussion of the recapitula- 

 tion doctrine, of which there is little left when the 

 author has done. We cannot help feeling, however, 

 ih.it tlnre is sure to be a rebound in a few years to 

 some subtler rehabilitation of Haeckel's famous 

 biogenetic law. The author believes in a good deal 

 of polyphyletism, and he confesses himself a muta- 

 tionist : transformist theories do not please him: 

 "C'est Involution avec ses brusqueries et ses diver- 

 gences qui constitue la r6alite\" 



THE PRINCIPLE OF RELATIVITY. 

 Das Relativitatsprinzip. By Dr. M. Laue. Pp. x + 

 208. (Braunschweig: F. Vieweg und Sohn, 1911.) 

 Price b.50 marks. 



IT is almost impossible nowadays to glance through 

 a journal containing" original papers in physics 

 without coming across something relating to the 

 Principle of Relativity. This principle is an extension 

 of that Newtonian relativity which enables us to treal 

 machines on a moving earth as if they were at rest. 

 The new extension covers the phenomena of optics, 

 heat, and electromagnetism. It is sometimes called 

 the electromagnetic principle of relativity, but as it 

 contains also a mechanical principle it has now be- 

 come usual to term it simply the Principle of Relati- 

 vity. It asserts that physical phenomena generally do 

 not depend upon rectilinear uniform translation 

 through space; that, for instance, the optical isotropy 

 of space is not affected by motion through it; that the 

 velocity of light is the same in all directions and 

 NO. 2187, VOL. 87] 



independent of displacement; and that it is therefore 

 impossible to discover, say, the earth's motion of trans- 

 lation by any optical, electrical, or mechanical device. 

 In fact, it is based upon the negative result of the 

 Michelson-Morley experiment, and all other attempts 

 to discover "asther-drift." 



Einstein, who founded the modern relativity theory 

 in 1905, based his arguments upon the impossibility 

 of establishing an absolute time-scale, either as re- 

 gards rate or as regards epoch, so long as the utmost 

 limit of rapidity of signalling is imposed by the finite 

 velocity of light. He showed how this limitation 

 affects all measurements of length and time whenever 

 the relative velocity dealt with approaches the velocity 

 of light. The clocks in a moving system, synchronised 

 by light signals, necessarily have a slower rate than 



those in a system at rest, in the ratio y/i — -j-:i, 



where v is the relative velocity and c the velocity of 

 light, and this applies whichever of two systems is 

 regarded as being at rest. There is, in fact, no 

 "absolute" time-scale. 



Many conclusions from this principle appear far- 

 fetched, even fantastic. Thus, not only electrons, but 

 all matter possesses an infinite "mass" when moving 

 with the velocity of light ; mass is identical with latent 

 energy; two particles projected in opposite directions 

 with the velocity of light have a "relative" velocity 

 which simply equals the velocity of light, and so on. 

 In spite of such apparent absurdities, the Principle 

 of Relativity has made what is no less than a tri- 

 umphal march through the world's physical publica- 

 tions. 



Dr. Laue's work comes, therefore, as a welcome 

 contribution to what has become a matter of very 

 living interest. He goes fully into the negative results 

 of Michelson, Trouton, Brace, Rayleigh, and others, 

 the positive results of Wilson, Rowland, Eichenwald, 

 Lebedew, Poynting (misspelt "Pointing"), and 

 Fizeau. and the theoretical w-ork of Lorentz, Einstein, 

 and Minkowski. He shows that there is no physical 

 evidence against the principle, and that it has the 

 advantage over other systems of accounting for the 

 absence of aether-drift. In the analytical work, a 

 vector algebra on the basis of Heaviside's notation 

 is used, but it is made, after Minkowski's example, 

 four-dimensional. A brief summary of operations 

 >wth these 'world-vectors" is of great assistance to 

 the reader. E. E. F. 



OUR BOOK SHELF. 



The Principles of Electric Wave Telegraphy and 

 Telephony. By Prof. J. A. Fleming, F.R.S. 

 Second edition (revised and extended). Pp. xx + 

 <3o6 . (London: Longmans, Green and Co., 1910.) 

 Price 285. net. 

 When reviewing the first edition of Prof. Fleming's 

 book five years ago we pointed out that it filled 

 to perfection the want for a thorough and exhaus- 

 tive treatise on the subject of wireless telegraphy, 

 and was sure of a warm welcome on that account. 

 Since then the volume has been twice reprinted, 

 and now there is issued a new edition largely re- 

 written and considerably improved. The rapid pro- 



