FUNDAMENTAL RELATIONS OF ANIMALS 35 



that fishes will be found in those few fossiliferous beds of the Si- 

 lurian System in which thus far they have not yet been found.*^ 

 Upon land we find equally everywhere Vertebrata, Articulata, and 

 Mollusks, but no Radiata, this whole branch being limited to the 

 waters; but as far as terrestrial animals extend, we find representa- 

 tives of the other three branches associated, as we find them all four 

 in the sea. Classes have already a more limited range of distribution. 

 Among Radiata, the Polypi, Acalephs, and Echinoderms are not only 

 all aquatic, they are all marine, with a single exception,^^ the genus 

 Hydra, which inhabits fresh waters. Among Mollusks, the Acephala 

 are all aquatic, but partly marine and partly fluviatile, the Gastero- 

 poda partly marine, partly fluviatile and partly terrestrial, while all 

 Cephalopoda are marine. Among Articulata, the Worms are partly 

 marine, partly fluviatile, and partly terrestrial, while many are in- 

 ternal parasites, living in the cavities or in the organs of other ani- 

 mals; the Crustacea are partly marine and partly fluviatile, a few 

 are terrestrial; the Insects are mostly terrestrial or rather aerial, yet 

 some are marine, others fluviatile, and a large number of those, which 

 in their perfect state live in the air, are terrestrial or even aquatic 

 during their earlier stages of growth. Among Vertebrata the Fishes 

 are all aquatic, but partly marine and partly fluviatile; the Reptiles 

 are either aquatic or amphibious or terrestrial, and some of the 

 latter are aquatic during the early part of their life; the Birds are all 

 aerial, but some more terrestrial and others more aquatic; finally, 

 the Mammalia, though all aerial, live partly in the sea, partly in fresh 

 water, but mostly upon land. A more special review might show that 

 this localization in connection with the elements in which animals 

 live has a direct reference to peculiarities of structure of such im- 

 portance, that a close consideration of the habitat of animals within 

 the limits of the classes might in most cases lead to a very natural 

 classification.^® But this is true only within the limits of the classes, 

 and even here not absolutely, as in some the orders only, or the 

 families only are thus closely related to the elements; there are even 

 natural groups in which this connection is not manifested beyond 



" See above, Sect. vii. 



^ I need hardly say in this connection that so-called fresh-water Polyps, Alcyonella, 

 Plumatella, etc., are Bryozoa, and not true Polyps. 



" Agassiz, "The Natural Relations between Animals and the Elements in which They 

 Live," American Journal of Science, IX (2d ser., 1850), 369-394. 



