352 THE CLASSICAL TRADITION 



law than either Keibel or Oppel, though he agrees that it 

 cannot be used for the construction of ancestral trees. But 

 he ascribes to it as a fact of development considerable 

 importance. The following passage gives a good summary 

 of his view as to the scope and validity of the law. "The 

 biogenetic law has not been shaken by the attacks of its 

 opponents. The assertion is still true that individual urgano- 

 genesis is exclusively dependent on phylogeny. But we 

 must not expect to find that all the stages in the develop- 

 ment of the separate organs, which coexisted in any member 

 of the phylogenetic series, appear at the. same tnne in the 

 individual ontogeny of the descendants, because each organ 

 possesses its own specific rate of development. In this way 

 it comes about naturally that organs which become differenti- 

 ated rapidly, as, for example, the medullary tube, as a rule 

 dominate earlier periods of ontogeny than do the organs of 

 locomotion, For the same reason the cerebral hemispheres 

 of man are almost as large in youth as in maturity. The 

 picture which an embryo gives is not a repetition in detail of 

 one and the same phylogenetic stage ; it consists rather of 

 an assemblage of organs, some of which arc at a phyletically 

 carl}' stage of development, while others are at a phyletically 

 older stage." 1 



A different line of attack was that adopted by O. Hertwig 

 in a series of papers, which contain also what is perhaps the 

 best critical estimate of the present position and value of 

 descriptive morphology. - 



It had not escaped the notice of many previous observers 

 that quite early embryos not infrequently show specific 

 characters even before the characters proper to their class, 

 order and genus are developed in direct contradiction of 

 the law of von Baer. Thus L. Agassi/. :; had remarked 



1 Quoted by Keibel (p. 790, 1897) from the niomechanik. 



/<*/< /.die und die Geivcbe, Jena, 1898, and the subsequent editions 

 of this text-book, published under the title of Allgemcinc liioloj^ie. 

 /)ie Entwickelung der Hioh^ie ii ncunzchntcn fahrhuniiert, Jena, 1900, 

 2nd cd., 1908. " Ueber die Stellung der vergl. Entwickelunslehre ?.ur 

 vcr-l. Anatomic, zur Systematik und Dcscendcn/theorie," Ilandh. 

 >\ /><>-. Entviickelvngslehre der \\'h-l>cliierc. iii., 3, pp. 149-80, Jena, 

 i'o6, b). Also in 1't. I. of Vol. I. (1906, a). 



-/// Essay on Classification, I <>nd<m, 1859. 



