SIGNIFICANCE OF MORPHOLOGY. 35 



similar that "the individual development is a pro- 

 gressive change from a more general form to a more 

 specific one." 



3. Influence of Darwin's theory. Fritz Miiller, 

 " Fur Darwin," Leipzig, 1864 ; Recognizes the full 

 significance of the facts hitherto accumulated, and 

 points out their relation as proving the theory, of de- 

 scent. The developmental processes of the individual 

 a more or less complete recapitulation of the develop- 

 mental history of the species, complicated and short- 

 ened by secondary variations due to adaptation, which 

 have been acquired in the struggle for existence. 



4. HseckePs " Biogenetisches Grundgesetz " ; "Die 

 Ontogenie ist eine kurze wiederholung der Phylogenie." 



III. Complications of the embryonic history to be 

 distinguished. 



1. Palingenetic characters which indicate ances- 

 tral structure ; e. </., the gill and aortic arches of 

 vertebrates. 



2. Coenogenetic characters, which have arisen as 

 larval or embryonic modifications ; e. g., the nauplius 

 stage of Crustacea. 



The principle of acceleration. 



IV. Examples. The trochophore larva of mollusks 

 and annelids ; its relations to the ccelenterates. The 

 larval history of balanoglossus and the ascidians. 

 Common stages in the vertebrate group as indicating 

 genetic relationship. 



