The Recapitulation Theory in Biology 9 



" (1) That the general of a large animal group expresses itself 

 earlier in the embryo than the special. (2) Out of the most 

 general of the structural relations evolves the less general, and 

 so forth until finally the most special appears. (3) Each em- 

 bryo of a particular animal form instead of passing through the 

 other particular forms, separates itself rather from them. (4) 

 In reality accordingly the embryo of a higher form is never like 

 to another animal form, but only to its embryo." 10 



Morgan illustrates these laws as follows: 



"For example two similar species of pigeons will follow the 

 same method of development up to almost the last stage of their 

 formation. The embryos of these two forms will be practically 

 identical until each produces the special characters of its own 

 species. On the other hand two animals belonging to different 

 families of the same phylum will have only the earlier stages in 

 common. Thus, a bird and a mammal will have the first stages 

 similar, or identical, and then diverge, the mammal adding the 

 higher characters of its group. The resemblance is between 

 corresponding embryonic stages and not between the embryo 

 of the mammal and the adult form of a lower group. 



"Von Baer was also careful to compare embryos of the same 

 phylum with each other, and states explicitly that there are no 

 grounds for comparison between embryos of different groups. 



. . In one place von Baer raises the question whether the 

 egg may not be a form common to all the phyla." u 



The phyla alluded to are, of course, the four great groups or 

 types. It is important t9 remember that von Baer had reference 

 in his statements to likenesses among existing animals, and not 

 to ancestral relationships. Von Baer did not accept evolution 

 although the issue between the two theories had been well 

 drawn before his death. Later Herbert Spencer pointed out 

 that a thoroughgoing application of von Baer's law in filial and 

 ancestral relationships implied the recapitulation of Oken, that 

 is, the idea that the embryogeny "repeated the story of the 

 creation of the animal groups." 



5. The Correspondence Stated as between Developmental Stages of 

 Existing Animals and the Succession of Adult Forms of Extinct 

 Representatives. Agassiz. 



Von Baer's significant contribution to the discussion did not 

 exert an immediate influence. It was Louis Agassiz, a pupil of 

 Cuvier, the principal originator of the type theory, who made 



l Quoted by Montgomery, toe. cit., p. 178. 

 "Loc. cit., p. 61. 



