28 ESSAY ON CLASSIFICATION. Part I. 



above Eadiata, as both stand below Vertebrata, but constructed upon plans expressing 

 difterent tendencies. To ajJi^reciate more ^^r^cisely these most general relations 

 among the great types of the animal kingdom, will require deeper investigations into 

 the character of their plan of structure than have been made thus far.^ Let, how- 

 ever, the respective standing of these great divisions be what it may ; let them differ 

 only in tendency, or in 2:)lan of structure, or in the height to which they rise, 

 admitting their base to be on one level or nearly so, so much is certain, that in 

 each type there are representatives exhibiting a highly complicated structure and 

 others which appear very simple. Now, the very fact that such extremes may be 

 traced, within the natural boundaries of each type, shows that in whatever manner 

 these great types are supposed to follow one another in a single series, the highest 

 representative of the preceding type must join on to the lowest representative of 

 the following, thus bringing necessarily together the most heterogeneous forms.^ It 

 must be further evident, that in proportion as the internal arrangement of each great 

 type will be more perfected, the greater is likely to appear the difference at the two 

 ends of the series which are vdtimately to be brought into connection with those of 

 other series, in any attempt to establish a single series for all animals. 



I doulit whether there is a naturalist now living who could object to an arrange- 

 ment in which, to determine the respective standing of Eadiata, Polyps would be 

 placed lowest, Acalephs next, and Echinoderms highest; a similar arrangement of 

 Mollusks would bring Acephala lowest. Gasteropoda next, and Cephalopoda highest; 

 Articulata would appear in the following order: Worms, Crustacea, and Insects, and 

 Vertebrata, with the Fishes lowest, next Reptiles and Birds, and Mammalia highest. 

 I have here purjjosely avoided every allusion to controverted points. Now if Mol- 

 Ivisks were to follow Eadiata m a simple series, Acephala should join on to the 

 Echinoderms; if Articulata, Worms would be the connecting link. We should then 

 have either Cephalopods or Insects, as the highest term of a series beginning with 

 Eadiata, followed by MoUusks or by Articulates. In the first case, Cephalopods 

 woiUd be followed by Worms ; in the second. Insects by Acephala. Again, the con- 

 nection with Vertebrata would be made either by Cephalopods, if Articvdata were 

 considered as lower than Mollusks, or hy Insects, if Mollusks were placed below 

 Articulata. Who does not see, therefore, that in proportion as our knowledge of the 

 true affinities of animals is improving, we accumulate more and more convincing 

 evidence against the idea that the animal kingdom constitutes one simple series? 



^ I regret to be unable to refer here to the con- between Progressive, Embryonic, and Prophetic 



tents of a course of lectures which I delivered upon Types, Proc. Am. Assoc, for 1849, p. 432. 

 this subject, in the Smithsonian Institution, in 1852. ^ Agassiz, (L.,) Animal Morphology, Proc. Am. 



Compare, meanwhile, my paper, On the Differences Assoc, for 1840, p. 415. 



