8 The Recapitulation Theory and Human Infancy 



Thought in the Nineteenth Century." 8 Only its more general 

 character need be noted here. It was, in brief, the logical culmi- 

 nation of the progress of descriptive science following the Re- 

 vival of Learning. From independent beginnings, embryology 

 and physiology, comparative anatomy, geology and geography, 

 and botany, had by degrees and in unrelated ways accumulated 

 a vast amount of unorganized knowledge which by the first 

 quarter of the nineteenth century invited a general and unified 

 formulation. The outcome was a conception constituted in 

 part of inductions and analogies derived from the observation 

 of nature, in part of the literal biblical interpretation of the 

 overshadowing Puritanism of the time. 9 



In this theory the plan of arranging the animal groups in 

 a single scale or series gave way to a system in which the species 

 were regarded as modifications of four main " types" or "em- 

 branchements," independent and equivalent " plans of struc- 

 ture," namely, the vertebrate, articulate, radiate, and mol- 

 luscan. The species representing these great types were 

 "fixed/ 7 in the sense that they had been separately created 

 as they were found to be, and were in no wise genetically related. 

 The study of fossils and geological formations led to the addi- 

 tional idea that species had suffered extinction coincident with 

 colossal revolutions or catastrophes in the life of the planet, 

 succeeded by the creation of new types. Only by the intervention 

 of such episodes could be explained the succession of life forms 

 so unequivocally indicated in these phenomena. 



4. Von Baer's Law: "The Law of Embryonic Resemblance. 11 



Von Baer was identified with the theory of types; in fact he 

 seems to have had some share in its formulation, although he 

 did not go to the extremes of its chief exponents. His views of 

 the significance of embryonic development are therefore to be 

 understood with this theory as a background. For von Baer 

 did not merely criticize the prevailing ideas of Meckel and others 

 on this subject. A distinguished embryologist, he held original 

 and striking ideas of his own, which had a large place in the 

 subsequent history of the discussion. Von Baer's position is 

 represented by the following laws, which he formulated: 



Vol. II, Chap. IX. 



Poulton, Essays on Evolution, pp. 56, 57. 



