PHYLOGENY 255 



Because of the incompleteness of our knowledge of pre-Cam- 

 brian life, the evidences of phylogeny of the lower forms must 

 be sought elsewhere than in the fossil remains. In individual 

 development, there is furnished a clue to racial history. 



Law of Biogenesis or Recapitulation Theory. — Since the days 

 of von Baer, it has been commonly observed that individuals 

 which are markedly different in appearance as adults have stages 

 in their development when the embryos have much the same 

 appearance. Structures common in the embryos may have 

 entirely different fates in the adults. The works of Haeckel 

 did much to popularize this observation. So generally did the 

 principle seem possible of application that it became expressed as 

 a law — the law of biogenesis- — ^which states that ontogeny is a 

 brief recapitulation of phylogeny. 



Thus, since the course of individual development more or less 

 faithfully repeats racial history, those characters which make 

 their appearance early in the course of embryology represent a 

 heritage from distant ancestors of the race. The more distant 

 the ancester the more numerous the offspring and consequently 

 the more kinds of animals which would display these ancestral 

 characters in the course of their development. Such ancestral 

 characters, appearing early in development, have been desig- 

 nated as palingenetic. In contrast with them, the coenogenetic 

 characters which appear relatively late in development are more 

 nearly specific and are found in much smaller groups of 

 individuals. 



There are many instances in which individual development has 

 become so highly modified that the operation of the biogenetic 

 law seems to be invalidated. In some instances, the processes of 

 ontogeny become so much shortened that whole chapters in the 

 racial history are deleted in the course of individual genesis. 

 These and other facts have led some investigators to discredit 

 the law entirely. It seems probable, however, that in many 

 instances valuable light is thrown upon racial history through 

 the study of embryology. 



Gastraea Theory. — Since a gastrula stage occurs in the onto- 

 geny of practically all of the Metazoa, Haeckel propounded a 

 theory which endeavored to establish a blood relationship among 

 all Metazoa through an ancestral form to which he applied the 

 name of the Gastraea. The Gastraea he considered as a hypo- 

 thetical form which in its adult organization displayed the 



