46 ESSAY ON CLASSIFICATION. Part 1. 



in Algiers and New Holland; none are known with three toes to the fore feet and 

 two to the hind feet. Those with three toes to the four feet inhal^it Europe, North- 

 ern Africa, and New Holland. There are none with three and four toes, either in 

 the fore feet or in the hind feet. Those with four toes to the four feet live in 

 New Holland; those with five toes to the fore feet and four to the hind feet, in 

 Bengal, and with four toes in the fore feet and five in the hind feet, in Africa, 

 the West Indies, the Brazils, and New Holland. Those with five toes to all four 

 feet have the widest distribution, and yet they are so scattered that no single zoolog- 

 ical province presents any thing like a complete series ; on the contrary, the mixtiu-e 

 of some of the representatives with perfect feet with others which have them rudi- 

 mentary, in almost every fauna, excludes still more decidedly the idea of an influence 

 of physical agents upon this development. 



Another similar series, not less striking, may be traced among the Batrachians, 

 for the characters of which I may refer to the works of Holbrook, Tschudi, and 

 Baird,^ even though they have not presented them in tliis connection, as the charac- 

 teristics of the genera will of themselves suggest their order, and further details upon 

 this subject would be superfluous for my purpose, the more so, as I have already 

 discussed the gradation of these animals elsewhere.^ 



Similar series, though less conspicuous and more limited, may be traced in every 

 class of the animal kingdom, not only among the living types, but also among the 

 representatives of past geological ages, which adds to the interest of such series in 

 showing, that the combinations include not only the element of space, indicating 

 omnipresence, Imt also that of time, which involves prescience. The series of Crinoids, 

 that of Brachiopods through all geological ages, that of the Nautiloids, that of 

 Ammonitoids from the Trias to the Cretaceous formation inclusive, that of Trilobites 

 from the lowest beds up to the Carboniferous period, that of Ganoids through all 

 formations; then again among living animals in the class of Mammalia, the series of 

 Monkeys in the Old World especially, that of Carnivora from the Seals, through the 

 Plantigrades, to the Digitigrades ; in the class of Birds, that of the Wading Birds, 

 and that of the Gallinaceous Birds; in the class of Fishes, that of Pleuronectidte and 

 Gadoids, that of Skates and Sharks ; in the class of Insects, that of Lepido]3tera from 

 the Tineina to the Papilionina ; in the class of Crustacea, that of the Decapods in 

 particular ; in the class of Worms, that of the Nudibi'anchiata or that of the Dorsibran- 



^ Holbrook, (.J. E.,) North American Her- Acad. Nat. Science, of Pliiladelisliia, 2d series, 



petology, Philadelphia, 1842, 4to. ; 5th vol. — vol. I., 1849, 4to. 



Tschudi, (J. J.) Classification der Batrachier, ^ Agassiz, (L.,) Twelve Lectures on Conipara- 



Neuchatel, 1838, 4to. — Baird, (Sp. F.) Revision tive Embryology, Boston, 1849, 8vo. ; p. 8. 

 of the North American Tailed Batrachia, Journal 



