Limitations and Adaptations. 15 



With the Articulata, we find another state of things. Two 

 of their classes, the worms and Crustacea, are chiefly marine, 

 or at least aquatic, as we have a number of fresh-water 

 worms, and some fresh-water Crustacea. But insects are, for 

 the most part, chiefly terrestrial, feeding upon terrestrial 

 plants, at least in their full-grown condition ; though a large 

 number of these animals are fluviatile, and even some marine, 

 during their earlier periods of life. In the Vertebrata, the 

 adaptations are more diversified. Only one class of these 

 animals is entirely aquatic — the fishes ; and the number of 

 the marine species is far greater than that of the fresh- water 

 kinds. Among reptiles there are many which are aquatic, 

 either throughout life, or through the earlier period of their 

 existence. But, as if animal life rose to higher organization, 

 as it leaves the ocean to inhabit dry land or fresh waters, we 

 find that the greater number of the aquatic reptiles are 

 fluviatile, and but a few marine. This fact agrees wonder- 

 fully with the natural gradation of the classes already men- 

 tioned. The lower type of animals, the Badiata, is almost 

 exclusively marine. Among Mollusca, we have a greater 

 number of marine types, a large number of fluviatile species, 

 and fewer terrestrial, and these are the highest in their class. 

 Again, among Articulata, the lower classes, worms and Crus- 

 tacea, are marine, or at least fluviatile, whilst the highest 

 class, that of insects, is chiefly terrestrial or fluviatile, during 

 the earlier periods of their growth. Among the Vertebrata 

 we see the lowest form, that of fishes, entirely aquatic, and 

 the same rule applies partially to the reptiles ; but as the 

 class rises, the number of the fluviatile species is greater than 

 that of the marine types. Next, among birds, which by their 

 structure are exclusively adapted to live in the atmospheric 

 air, we find* the larger number to be terrestrial, and only 

 the lower ones to live upon water, or dive occasionally into 

 it, always seeking the surface, however, to breathe and to per- 

 form their most important vital functions. It is, neverthe- 

 less, not a little strange, that this class should by nature be 

 adapted to rise into the air, just as if the first tendency to- 

 wards liberating them from the aquatic element had been 



