No 437] NOTES AND LITERATURE. 



349 



opposite edge parallel with it." With these directions as an example 

 of clearness, the accounts of numerical aperture and polarized light 

 may best be imagined. Students will find the "too voluminous" 

 " works of reference " of Carpenter, Gage and Lee not rendered less 

 useful by the appearance of " Biological Laboratory Methods." 



H. VY. R. 



ZOOLOGY. 



Sexual Dimorphism. — Sexual dimorphism among animals and 

 the evolution of secondary sexual characters form the subject matter 

 of an interesting volume by Cunningham. 1 The author points out 

 the inadequacy of natural selection as an explanation of the very con- 

 stant characters upon which animal classification is based in that it 

 must be admitted that many of these characters are of no obvious 

 advantage to their possessors. He next turns his attention to sec- 

 ondary sexual characters and claims that here too that special form 

 of selection called by Darwin sexual selection is ineffective because 

 again the differences are not of a kind to afford a basis for the 

 selective process. In his opinion the origin of these characters has 

 been due to Lamarckian factors. It must be admitted that the 

 influence of the environment profoundly changes animals. Those 

 changes that occur at the breeding season are dependent upon the 

 changes of habits characteristic of that period. They consequently 

 form the basis for the evolution of secondary sexual characters. 

 That these changes may be inherited is well known, hence we should 

 not deny that they are examples of the inheritance of acquired charac- 

 ters because we are ignorant of the method by which their inheritance 

 is accomplished. From this standpoint the author reviews a large 

 range of secondary sexual characters from the mammals to the low- 

 est metazoa in which such characters are known. While the line of 

 argument will probably not be convincing to even the milder Weis- 

 mannians, the wide range of illustrations brought together by the 



Black, 1900. Svo, xii-f 317 pp., figs. 



