September 22, J905.J 



SCIENCE. 



369 



photography by Lippmann (1891, 1892), 

 showing that the electric and not the mag- 

 netic vector is photographically active. 



The theory of interferences from a 

 broader point of view, and including the 

 occurrence of multiple reflections, was suc- 

 cessively perfected by Poisson (1823), 

 Fresnel (1823), Airy (1831). It has re- 

 cently been further advanced by Feussner 

 (1880, et seq.), Sohncke and Wangerin 

 (1881, 1883), Rayleigh (1889) and others. 

 The interferences along a caustic were 

 treated by Airy (1836), but the endeavor 

 to reconstruct geometric optics on a diffrac- 

 tion basis has as yet only succeeded in cer- 

 tain important instances, as already men- 

 tioned. 



Carl Barus. 

 Brown University. 



{To be continued.) 



SCIENTIFIC BOOKS. 



Species and Varieties, their Origin hy Muta- 

 tion. By Hugo de Vries; edited by 

 Daniel Trembly MacDougal. Chicago, 

 The Open Court Publishing Co. 

 De Vries's great work ' Die Mutationsthe- 

 orie ' marks an epoch in biology as truly as 

 did Darwin's ' Origin of Species.' The revo- 

 lution that it is working is less complete, per- 

 haps, because there has remained no such im- 

 portant doctrine as that of continuity to be 

 established. But there was need of a revolu- 

 tion in our method of attacking the problems 

 of evolution. Ever since Darwin's time most 

 biologists have been content to discuss and 

 argue on the ntiodus operandi of evolution. 

 The data collected by Darwin have been 

 quoted like scriptural .texts to prove the truth 

 of the most opposed doctrines. We have seen 

 biologists divided into opposing camps in de- 

 fense of various isms, but of the collection of 

 new data and, above all, of experimentation 

 we have had little. The great service of de 

 Vries's work is that, being founded on experi- 

 mentation, it challenges to experimentation as 

 the only judge of its merits. It will attain 

 its highest usefulness only if it creates a wide- 



spread stimulus to the experimental investiga- 

 tion of evolution. 



To be read, nowadays, a book must be brief. 

 Much of the success of the ' Origin of Species ' 

 was due to the mass of material which was 

 left out. The very bulkiness of de Vries's 

 original work must prevent its being read as 

 widely as it deserves. There was needed a 

 briefer presentation of de Vries's views and 

 one in English, and this need has fortunately 

 been filled by de Vries himself in the work 

 now under review. This book should be read 

 with care by every biologist ; the brief synopsis 

 of its contents which alone is possible here 

 can in no wise raake such a reading unneces- 

 sary. 



After an introductory chapter, the funda- 

 mental distinction between elementary species 

 and varieties is discussed. Elementary species 

 are forms that are distinct in several characters 

 from their close allies and breed true. They 

 are thought to have arisen from their parent 

 form in a progressive way, i. e., by the addi- 

 tion of a new characteristic. In this they are 

 distinguished from retrograde varieties on the 

 one hand and mere fluctuations of characters 

 on the other. 



The subject of retrograde varieties (consti- 

 tuting the third section of the book) assumes 

 great importance in de Vries's system. They 

 are varieties in the new (restricted) sense. 

 They differ from the parent species usually 

 in the absence of some single character; for 

 example, the white flower variety of a plant 

 or the hairless form of an ordinarily hirsute 

 species. The eliminated characters are of a 

 few, definite, constantly recurring kinds. In 

 this ^'espect varieties differ from elementary 

 species whose differential characters are most 

 varied. Moreover, varieties are subordinate 

 to some elementary species, whereas elemen- 

 tary species are coordinate. 



In self-fertilization varieties behave in a 

 characteristic way. They are frequently con- 

 stant. Even varieties that are intermediate 

 between the parent species and other varieties 

 may be as stable as the latter. Indeed, we 

 know that certain garden varieties have been 

 bred in their present form for two or three 

 centuries. Such varieties may, however. 



