FUNDAMENTAL RELATIONS OF ANIMALS 31 



four great branches of the whole animal kingdom; for, however plain 

 the inferiority of the Radiata may seem when compared with the 

 bulk of the Mollusks or Articulata, or still more evident when con- 

 trasted with the Vertebrata, it must not be forgotten that the struc- 

 ture of most Echinoderms is far more complicated than that of any 

 Bryozoon or Ascidian of the type of Mollusks, or that of any Hel- 

 minth of the type of Articulata, and perhaps even superior to that 

 of the Amphioxus among Vertebrata. These facts are so well ascer- 

 tained, that an absolute superiority or inferiority of one type over 

 the other must be unconditionally denied. As to a relative superior- 

 ity or inferiority, however determined by the bulk of evidence, 

 though it must be conceded that the Vertebrata rank above the three 

 other types, the question of the relative standing of Mollusks and 

 Articulata seems rather to rest upon a difference in the tendency of 

 their whole organization than upon a real gradation in their struc- 

 ture; concentration being the prominent trait of the structure of 

 Mollusks, while the expression "outward display" would more 

 naturally indicate that of Articulata, and so it might seem as if 

 Mollusks and Articulata were standing on nearly a level with one 

 another, and as much above Radiata as both stand below Vertebrata, 

 but constructed upon plans expressing different tendencies. To ap- 

 preciate 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.^^ Let, however, the respective standing of these great divisions 

 be what it may; let them differ only in tendency or in plan of struc- 

 ture 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 ex- 

 tremes 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 



" Compare my paper, "Progressive, Embryonic, and Prophetic Types," Proceedings, 

 AAAS, II (1850), 432-438. 



"Agassiz, "Animal Morphology," Proceedings, AAAS, II (1850), 415. 



