Chap. III. 



EMBRYOLOGICAL SYSTEMS. 



225 



beginning and controls the ■whole development. The embrj-o of the Vertebrate 

 is a Verte])rate from the beginning, and does not exhibit at any time a corre- 

 spondence with the Invertebrates. The embryos of Vertebrates do not pa.ss in 

 their development through other permanent types of animals. The fundamental 

 type is first developed, afterwards more and more subordinate characters appear. 

 From a more general type, the more special is manifested, and the more two forms 

 of animals differ, the earlier must their development be traced back to discern 

 an agreement between them. It is barely possible that in their first beginning 

 all animals are alike and present only hollow sjiheres, but the individual develop- 

 ment of the higher animals certainly does not pass through the permanent forms 

 of lower 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 mo.st 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 imlike them. An embryo of a higher type 

 is, therefore, never identical with another animal type, but only with an embryo. 

 Thus far do the statements of von Baer extend.^ It is evident from thi.s, that 

 he has clearly perceived the limitation of the different modes of embrj^onic develop- 

 ment within the respective bi'anches of the animal 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. 



* The account which Iluxley gives of Baer's 

 views, (see Baden Powell's Essays, Appendix 7, 

 p. 495.) is incorrect. Baer did not "demonstrate 

 tliat the classification of Cuvier was, in the main, 

 simply the expression of the fact, that tliere are 

 certain common plans of development in the animal 

 king<lom," etc., for Cuvier recognized these plans in 

 the structure of the animals, before Baer traced 

 their development, and Baer himself protests against 

 an identitication of his views with those of Cuvier. 

 (Baer's Entwiek., p. 7.) Nor has Baer demon- 

 strated the " doctrine of the unity of organization 

 of all animals," and jjlaced it " ui)on a footing .as 

 secure as the law of gravitation," and arrived at "the 

 grandest law," that, up to a certain point, the develop- 

 ment "followed a plan common to all animals." On 

 the contrary, Baer admits four distinct types of 

 animals, and four modes of development. He only 



29 



adds : " It is barely possible that in their first begin- 

 ning all animals are alike." Iluxley must also 

 have overlooked Cuvier's introduction to the " Regne 

 Animal," (2d edit., vol. 1, p. 48, quoted verbatim 

 above, p. 193,) when he stated that Cuvier "did not 

 attempt to discover upon what plans animals are con- 

 structed, 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 Baer's merit consists 

 in having discovered 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." 



