his first embryo logical investigation led Baer to conclude that 

 in chick development the features of the vertebrate animal 

 appear very early, he had already referred to his doubts 

 in his dissertation published five years earlier.' 



The similarity between the individual embryonic stages 

 and the adult stages of others Baer considered as unquestion- 

 able, but without major significance. Against the view that 

 development of the individual passes through constant forms, 

 which are characteristic of lower animals, Baer promoted the 

 following objections. If it were true, there should not be 

 in the embryo any characters that are absent in lower adults 

 (for example, the reserve of nutritional materials in the 

 yolk or the intestinal loops hung externally). Besides, 

 there might be similarity in just one feature or aggregate of 

 features. According to the structure of the heart and 

 extremities, bird embryos are similar to those of fish; 

 but the other series of features inherent to early fish 

 development are not present. The similarities of embryos 

 with animals which are more highly organized also argues 

 against the recapitulation idea . Finally, those organs 

 which are inherent only tc the higher animals appear very 

 early. For example, the foundation of the vertebral column 

 appears when the vertebrate animal, according to the idea 

 of repetition of the species' history, would only have 

 passed the stage of invertebrates (I, Sch. V 2b, 204 - 206). 



All of Baer's argument against the repetition of the 

 species history in ontogenesis, he mobilized for his theory 

 of types. The theory of types played, in the history of 

 science, a dual role: a positive one, insofar as it represented 

 the basic natural classification system, and a negative role, 

 insofar as the theory of tyr^s represented an antithesis to 

 the theory of one origin, the evolution of the organic world. 

 To the theory of types, Engels' familiar words relate directly: 

 "... from that time, when they began working in biology in 

 the light of evolutionary theory, one rigid boundary line for 



7. Baer, DE FOSSILIBUS ANIMALIUM RELIOUIIS IN 

 PRUSSIA REPERTIS DISSERTATIO. Regiomontii , 

 1823, 40 pp. (98) . 



350 



