740 



THE AMERICAN NATURALIST 



[Vol. XL 



in the case of heliotropism, galvanotropism, and ehemotropism, but 

 geotropism and stereotropism are more difficult to explain. The 



arc made clear. The discussion of the control of the degree and the 

 sense of heliotropic response is especially interesting. The demon- 

 stration of the possibility of controlling these reactions by chemical 

 and other means, the apparent uselessness of many of them, and the 

 fact that many could never be used at all in Nature tend to show that 

 they could not have been acquired by way of natural selection. 



The chapters on fertilization and heredity will probably be the 

 first to be read by many biologists. Two effects of the entrance of a 

 spermatozoon into an egg must be distinguished. The first is the 

 starting of the process of development, the developmental effects, 

 while tiie second is the transmission of the |)aternal qualities to the 

 organism, the hereditary (.-irc r. I iidrr the first of these heads are 

 considered the specific chann k r of the fci iiliziug power of the sperma- 



ciples ami rosuhs of cxiMTiuirnis in nrtiiicial parthenogetiesis. While 

 the development of tlic t'u^- may be caused without the presence of 

 a sperm nucleus or an much^-iKMl fraiitnciii of cgi: |)r(>t()])lasm may be 



nize the hernhtJrv elVcct of ru- or .pcnii micl.Mi. a. a .li.stinct phe- 

 nomenon. The struetuiv of ihr -..nine. i. ^^.•..^,ii„^l^ simple ami 

 yet instincts as well as bo.lilv fnnn arc tiaii-niittc,! tlin.ugh the sexual 



very far here and we 'first tlmik of <lcfi.iitc chemical compounds as 



ments upon the toxic effect (.f the 1.1. .od ..f a differ(>nt species smns 



The minute size of the gametes prcclii.lc> any cxtciisiv.^ ajialvM.s of 



