PRINCIPAL SYSTEMS OF ZOOLOGY 251 



ones. What is common in a higher group of animals is always sooner 

 developed in their embryos than what is special; out of that which is 

 most general arises that which is less general, until that which is most 

 special appears. Each embryo of a given type of animals, instead of 

 passing through other definite types, becomes on the contrary more 

 and more unlike them. An embryo of a higher type is, therefore, 

 never identical with another animal type, but only with an em- 

 bryo.^^ 



Thus far do the statements of von Baer extend. ^^ It is evident from 

 this that he has clearly perceived the limitation of the different modes 

 of embryonic development within the respective branches of the ani- 

 mal kingdom, but it is equally certain that his assertions are too 

 general to furnish a key for the comparison of the successive changes 

 which the different types undergo within their respective limits, and 

 that he is still vaguely under the impression that the development 

 corresponds in its individualization to the degrees of complication 

 of structure. This could hardly be otherwise, as long as the different 



^' [This is a succinct and entirely accurate statement of von Baer's views on the re- 

 capitulation concept, which Darwin would have profited from had he been fully aware 

 of the problem. For a full exposition of the relationship of Darwin, von Baer, and 

 Agassiz to the recapitulation idea, see Jane Oppenheimer, "An Embryological Enigma 

 in the Origin of Species," in Bentley Glass, et al. (eds.). Forerunners of Darwin, 1745- 

 1859 (Baltimore, 1959), pp. 292-322.] 



^ The account which Huxley gives of von Baer's views, (see Powell, Essays, Appendix 

 7, p. 495) is incorrect. Von Baer did not "demonstrate that the classification of Cuvier 

 was, in the main, simply the expression of the fact, that there are certain common 

 plaiis of development in the animal kingdom," etc., for Cuvier recognized these plans 

 in the structure of the animals, before von Baer traced their development, and von 

 Baer himself protests against an identification of his views with those of Cuvier (£n- 

 tiuickelungsgeschichte, I, 7). Nor has von Baer demonstrated the "doctrine of the 

 unity of organization of all animals" and placed it "upon a footing as secure as the 

 law of gravitation," and arrived at "the grandest law" that, up to a certain point, the 

 development "followed a plan common to all animals." On the contrary, von Baer 

 admits four distinct types of animals and four modes of development. He only adds: 

 "It is barely possible that in their first beginning all animals are alike." Huxley must 

 also have overlooked Cuvier's introduction to the Regne animal, (2d ed.), I, p. 48, 

 when he stated that Cuvier "did not attempt to discover upon what plans animals are 

 constructed, but to ascertain in what manner the facts of animal organizations could 

 be thrown into the fewest possible propositions." On the contrary, Cuvier's special 

 object for many years has been to point out these plans and to show that they are 

 characterized by peculiar structures, while von Baer's merit consists in having dis- 

 covered four modes of development which coincide with the branches of the animal 

 kingdom, in which Cuvier recognized four different plans of structure. Huxley is 

 equally mistaken when he says that Cuvier adopted the nervous system "as the base 

 of his great divisions." 



