358 ESSAY ON CLASSIFICATION. 



defined from the beginning, and controls the whole deve- 

 lopment. The embryo of the Vertebrate is a Vertebrate 

 from the beginning, and does not exhibit at any time a 

 correspondence with the Invertebrates. The embryos of 

 Vertebrates do not pass, in their development, through 

 other permanent types of animals. The fundamental type 

 is first developed, and afterwards more and more subordi- 

 nate 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 spheres ; but the individual de- 

 velopment 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 most general arises that which is less gene- 

 ral, until that which is most special appears. Each em- 

 bryo 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 there- 

 fore never identical with another animal type, but only 

 with an embryo. 



Thus far do the statements of von Baer extend. 1 It is 



1 The account which Huxley gives protests against an identification of 



of Baer's views (see Baden Powell's his views with those of Cuvier. (Ba- 



Essays, Appendix 7, p. 495) is incor- er's Entwick. p. 7.) Nor has Baer 



rect." Baer did not "demonstrate demonstrated the "doctrine of the 



that the classification of Cuvier was, unity of organization of all animals," 



in the main, simply the expression and placed it " upon a footing as 



of the fact, that there are certain secure as the law of gravitation," 



common plans of development in the and arrived at "the grandest law," 



animal kingdom," etc.; for Cuvier that, up to a certain point, the deve- 



recognized these plans in the struc- lopmeut "folio-wed a plan common to 



ture of the animals before Baer traced all animals." On the contrary, Baer 



their development ; and Baer himself admits four distinct types of animals, 



