June 27, 1901] 



NA TURE 



209 



The O. alln'da, on the other hand, which appeared 

 frequently, is a weak plant, not very fertile, but perfectly 

 constant so far as it went. 



The last species in the above list, the CEnothcra scin- 

 tillans, differs from the others in this respect, that it is 

 extremely unstable, i.e. possesses the property of muta- 

 tion to a high degree, a large proportion of its descend- 

 ants belonging to other species, specially O. oblonga and 

 Lainarckiana itself 



Want of space prevents us from going into further de- 

 tails. Enough has been said to show that de Vries has 

 evidently made a momentous discovery. So far as his 

 observations go, new species appear suddenly by mutation., 

 never as the outcome of a progressive variatinn. With 

 legitimate pride the author declares that he has been able 

 for the first time to watch the formation and development 

 of new species. The facts are so striking and convincing 

 that an outsider like the reviewer cannot but feel that a 

 new period in the theories of the origin of species and of 

 evolution has been inaugurated. 



.-\s we saw, some of the new species which made their 

 appearance did not seem to be inferior in stability to the 

 mother-species ; on the other hand, one of the species, 

 the O. lata., only appeared as female plants without pollen, 

 and the O. albida did not show the same vitality as the 

 others and was evidently doomed to disappear again. 

 The observations, therefore, do not support the idea that 

 in the formation of new species Nature is carrying out a 

 definite plan ; on the contrary, it all looks like accident, 

 K new species may be one strong and fertile enough to 

 remain, and possibly, under favourable conditions, replace 

 the mother-species, but it may just as well be a sickly 

 kind without any chance in the struggle for e.xistence. 

 For the struggle for life between individuals de \'ries 

 substitutes the struggle for continued existence between 

 species, the new species always appearing suddenly. 



De \'ries' views are thus directly opposed to the com- 

 mon form of the theory of evolution ; not that the 

 importance of the single variations had escaped attention 

 altogether, but they were always lost sight of, and 

 prime importance is generally attached to the selection 

 through the ordinary variations. De Vries' experiments 

 support the results arrived at by Scott and other palfeonto- 

 logists that there is no evidence in the successive strata of 

 the earth of a gradual development of one species into 

 another and that everything points at small but sudden 

 transitions. 



It can hardly be believed that the species which de 

 N'ries happened to come across can be the only living one 

 possessing the property of mutation, and men of science 

 may therefore look forward to a new period of extensive 

 research on the lines of de Vries' work. One feels that 

 new life has been infused into the problem, and that 

 tangible facts are now available and experiments which 

 will replace a good deal of rather empty theorising and 

 hollow controversy between rival speculations. 



Turning to physical research we naturally think, first 

 of all, of the discovery made by Prof Zeeman some years 

 ago when still assistant at Leyden in Prof Onnes' labora- 

 tory. This discovery of the influence of a magnetic field 

 on the period and character of light radiated by a source 

 in the field came just at the right time to bring the theory 

 of ions or ; electrons into prominence, a theory the 

 necessity of which had already appeared in many ways, 

 and which had been worked out for the first time by 

 H. .A. Lorentz. In fact, without any calculation it is 

 easily seen what influence a magnetic field must have on 

 the light-vibrations, if these consist in the vibrations of 

 charged particles. We have only to resolve the vibra- 

 tion of the electron in the direction of the field and at 

 right-angles to it, and, again, the latter component into two 

 circular vibrations of opposite directions, to see at once 

 that the magnetic force must increase the centripetal ac- 

 celeration in the one and diminish it in the other circle 



NO. 1652, VOL. 64] 



without affecting the third \ibration ; thus the ordinary- 

 doublet in the direction of the field, and the triplet in a 

 direction at right-angles to it, both with their proper 

 states of polarisation, may be directly inferred from the 

 theory in its simplest form. The directiory of the circular 

 polarisation of the doublet shows the preponderance of 

 the negative over the positive electrons in producing the 

 phenomenon, and from the magnitude of the change in, 

 wave-length, i.e. the width of the doublet, the ratio of 

 the charge of the electrons to their mass can be inferred. 



It need not be here explained how these conclusions- 

 were confirmed by, and confirmed results obtair>ed in the- 

 study of the conduction of electricity in gases, by J. J. 

 Thomson and his pupils and others. Zeen>an.'s phe- 

 nomenon soon became public property, and has smce beert 

 developed by many others as well as by Zeeman himself. 



H. A. Lorentz, whose name is connected with Zeeman'& 

 discovery and its immediate explanation, published the- 

 first complete account of his electron-theory in 1892 in^ 

 French, and a more complete version in 1S9.5 in German- 

 (" Versuch einer Theorie der electrischen und optischen- 

 Erscheinungen in bewegten Korpern." Leyden ; Brill). 

 To this theory Lorentz was led by his discussion of aber- 

 ration phenomena ; there is, perhaps, no phenomenon' 

 which is so readily explained in elementary text-book* 

 but gives so much trouble when properly discussed as- 

 aberration. 



Lorentz's researches led him to adopt Fresnel's theory^ 

 which assumes that the ether is at rest and that bodies- 

 move through it without disturbing it. This theory is ir» 

 accordance with the negative results of Lodge's well- 

 known attempts to put the ether in motion by spinning 

 two heavy wheels close together in the same directior>. 

 Starting from this hypothesis and assuming that all- 

 electric phenomena — includmg light — in bodies are due 

 to the presence, motion and vibration of electrons actmg 

 on each other through the ether, Lorentz developed au 

 theory which leads to the proper Maxwell-equations for 

 bodies at rest, and, moreover, explains the great majority 

 of the experiments and phenomena relating to moving 

 bodies— such as aberration, Doppler's principle andi 

 Fresnel's law for the velocity with which light is "dragged 

 along " by a moving body through which it passes. 



In order to account for the negative result of Michel- 

 son's aberration-experiment, Lorentz assumes, as was 

 done independently by Fitzgerald, that a body moving 

 through the ether diminishes in dimension in the directioi* 

 in which it moves. 



Particular interest attaches to a further development of 

 the theory in the direction of an explanation of gravita- 

 tion on electro-magnetic principles. In a paper published 

 in 1900 Lorentz shows, first of all, that gravitation 

 cannot be explained by assuming that bodies are con- 

 stantly emitting electro-magnetic radiations of very- 

 short wave-length and high penetrating pwwer, and that 

 gravitation is due to the action of the ether in this- 

 disturbed condition on the electrons contained irk bodies^ 

 Lorentz therefore proposes a different theory which is, in> 

 a way, an adaptation of Mosotti's theory of gravitation. 

 Assuming that all bodies contain an equal number of 

 positive and negative ions, it is clear that an explanation- 

 of gravitation by the mutual action of these ions, this 

 action being, of course, transmitted by the ether vi-ith the 

 velocity of light, can only succeed if it is assumed that 

 in some way the condition of the field produced by a 

 positive ion differs from that which is due to a negative- 

 ion; the action of the fields of the two kinds of ions on other 

 ions is then supposed to be different, but in such a way 

 that the action of a -f field on a -1- or - ion is the same 

 as that of a - field on a - or -I- ion respectively. 

 Hence it follows that there will be no electrical action 

 between two bodies containing ions, i.e. no tendency 

 to separate the positive and negative ions, but a resultant 

 action which constitutes what is observed as gravitation. 



