GRADATION OF STRUCTURE AMONG ANIMALS. 39 



and so it might seem as if Mollusks and Articulata were 

 standing on nearly a level with one another, and as much 

 above Kadiata as both stand below Vertebrata, but con- 

 structed upon plans expressing different tendencies. To 

 appreciate more precisely 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. 1 Let, however, 

 the respective standing of these great divisions be what 

 it may ; let them differ only in tendency, or in plan 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, thus far, that in each type there are representa- 

 tives 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 necessarily bringing together the most heterogeneous 

 forms. 2 It must be further evident, that, in proportion 

 as the internal arrangement of each great type becomes 

 more perfected, the greater is likely to appear the differ- 

 ence at the two ends of the series, which are ultimately 

 to be brought into connection with one another in any 

 attempt to establish a single series for all animals. 



I doubt whether there is a naturalist now living who 

 would object to an arrangement in which, to determine 



1 I regret to be unable to refer here gressive, Embryonic, and Prophetic 



to the contents of a course of lectures Types; Proc. Am. Assoc. for 1849, 



which I delivered upon this subject, p. 432. 



in the Smithsonian Institution, in 3 AGASSIZ (L.), Animal Morpho- 



1852. Compare, meanwhile, my pa- logy; Proc. Am. Assoc. for 1849, p. 



per, On the Differences between Pro- 415. 



