DEVELOPMENT. 147 



tlius are less differentiated from the inorganic world than 

 animals. Though in those microscopic Trotophyta and Pro- 

 tozoa inhabiting the water — the spores of algae, the gemmules 

 of sponges, and the infusoria generally — we see locomotion 

 produced by ciliary action ; yet this locomotion, while rapid 

 relatively to the size of the creatures, is absolutely slow. Of 

 the Coelenterata, a great part are either permanently rooted or 

 habitually stationary ; and so have scarcely any self- mobility 

 but that implied in the relative movements of parts ; while 

 the rest, of which the common jelly-fish will serve as a sam- 

 ple, have mostly but little ability to move themselves through 

 the water. Among the nigher aquatic Invertebrata, — cuttle- 

 fishes and lobsters, for instance, — there is a very considerable 

 power of locomotion ; and the aquatic Vertebrata are, con- 

 sidered as a class, much more active in their movements than 

 the other inhabitants of the water. But it is only when we 

 come to air-breathing creatures, that we find the vital charac 

 teristic of self-mobility manifested in the highest degree. 

 Flying insects, mammals, birds, travel with a velocity far 

 exceeding that attained by any of the lower classes of ani- 

 mals ; and so are more strongly contrasted with their inert 

 environment. Thus, on contemplating the various 



grades of organisms in their ascending order, we find them 

 more and more distinguished from their inanimate media, in 

 structure, in form, in chemical composition, in specific gravity, 

 in temperature, in self -mobility . It is true that this general- 

 ization does not hold with complete regularity. Organisms 

 which are in some respects the most strongly contrasted with 

 the environing inorganic world, are in other respects less so 

 than inferior organisms. As a class, mammals are higher 

 than birds ; and yet they are of lower temperature, and have 

 smaller powers of locomotion. The stationary oyster is of 

 higher organization than the free-swimming medusa ; and 

 the cold-blooded and less heterogeneous fish, is quicker in its 

 movements than the warm-blooded and more heterogeneous 

 sloth. But the admission that the several aspects under 



