1899] MEHNERT'S PRINCIPLES OF DEVELOPMENT 391 



date of sexual maturity. Some ctenophore larvae are mature almost 

 immediately after they leave the egg, but this is not true of the eagle. 

 And though there are disadvantages, there are also some advantages on 

 the side of the eagles. There is opportunity for the body to influence 

 the germ-plasm, which nobody can deny. But having said so much we 

 need not follow the author into his discussion of premature sexuality 

 or neotaenia on the one hand, or of the advantages of late marriages 

 on the other. 



It must be confessed that the book we have been dealing with is 

 hard to read and harder to expound, and it is quite possible that we 

 have in various instances failed to catch the author's meaning. It is 

 not easy to estimate how much it really amounts to — this " principle 

 of organogenesis." But as our position in regard to the physiology of 

 development may be compared to that of travellers hesitating in thick 

 mist by the edge of a bog where path and pitfall are alike hidden, it 

 would seem that he must be either a very wise or a very foolish person 

 who takes it upon himself to dismiss lightly the suggestions of a 

 measurer. 



The University, 

 Aberdeen. 



