October 13, 1911] 



SCIENCE 



493 



(five-leaved clovers), Plantago lanceolata ra- 

 mosaj and variegated leaves). 



" If one of the two parents [in a cross] 

 stands in the relation to the other of having 

 arisen from it hy retrogressive or degressive 

 mutation," the progeny, in successive genera- 

 tions, follow Mendel's laws; otherwise they do 

 not, and the result of a cross is then a uni- 

 sexual, or constant hybrid race. Constant 

 hybrid races correspond to progressive muta- 

 tions, the Mendelian law to retrogressive and 

 degressive forms of differentiation (pp. 576- 

 577). 



But though the mutation theory is a direct 

 outgrowth of the hypothesis of intracellular 

 pangenesis, it fortunately does not stand or 

 fall with the latter, for no scientific theory 

 ever had a firmer foundation in fact — in ex- 

 perimental evidence — than that of mutation. 



De Vries claims to have demonstrated ex- 

 perimentally that : (1) " Ordinary or fluctu- 

 ating variability does not provide material for 

 the origin of new species " (p. 56) . While 

 this does not exclude the possibility of differ- 

 ent modes of origin of new species, still " In- 

 ferences drawn from data after its appearance 

 can hardly be considered as decisive" (p. 56). 

 That is, the proilem is an experimental one. 



(2) " The elementary species are demonstrably 

 the existing units of the system; whilst the 

 larger species are only aggregations of these." 



(3) Elementary species (the theory has noth- 

 ing to do, except indirectly, with the mode of 

 origin of taxonomic groups) do arise by the 

 method of mutation (discontinuous varia- 

 tion). They have time and again, in a wide 

 variety of sorts, been seen so to arise. No one 

 has ever actually witnessed the origin of a 

 species by any other method. 



It is probable that the doctrine of the ele- 

 mentary units of organisms will ultimately 

 prove to be de Vries's most important contri- 

 bution; though perhaps this doctrine means 

 less to the advancement of philosophical and 

 experimental biology than the firm establish- 

 ment of the fact, quoted on the fly-leaf of 

 " Species and Varieties," that " The origin of 

 species is an object of experimental investiga- 

 tion." The demonstration of this fact, so 



conclusively as to compel practically every 

 investigator to acknowledge its truth, is the 

 greatest service rendered to evolutionary biol- 

 ogy since Darwin. 



On the whole, the translators have per- 

 formed their work well, though in a few 

 places (e. g., on pp. 608 and 609, cf. German 

 edition. Vol. II., pp. 666), the English can 

 hardly be regarded as a translation of the 

 original. The color plates and the text-figures, 

 notably plate IV., and figures 26, 40 and 54, 

 are quite inferior to those of the German edi- 

 tion. However, it is not easy to find points 

 to criticize adversely, and every one is hoping 

 that an English edition of the now omitted 

 portions, by the same translators, will soon 

 appear. 



C. Stuart Gager 



The Brooklyn Institute 

 OF Arts and Sciences, 



Brooklyn Botanic Garden 



Outlines of the Theory of Electromagnetism. 

 By Gilbert T. Walker. Cambridge, Uni- 

 versity Press. 1910. 



This volume contains a series of lectures 

 delivered before the Calcutta University on 

 some of the more important developments of 

 electromagnetic theory. The chief novelty of 

 it, compared with other English books on 

 mathematical physics, lies in the consistent 

 use of vector methods, and their advantage is 

 shown by the large amount of material con- 

 densed into fifty-two pages. While little of 

 the material is new, the book wiU be of great 

 assistance to those who wish to familiarize 

 themselves with the present condition of the 

 theory, as well as to those who wish to obtain 

 a working knowledge of vector methods ap- 

 plied to physical problems. For the latter 

 object no better course could be devised than 

 a careful study of this book, with frequent 

 transformations of the vector formulae into 

 their more familiar Cartesian equivalents. 



The first chapter gives an outline of vector 

 analysis, including the vector expressions for 

 the more important analytical theorems of 

 constant use. In the second chapter vector 

 methods are applied to magnetostatics, and 



