DEVELOPMENT. 175 



degrees more than the heat of their medium. While mam- 

 mals and birds maintain a heat which continues almost un- 

 affected by external variations, and is often greater than that 

 of the air by seventy, eighty, ninety, and even a hundred 

 degrees. Once more, in greater self-mobility a pro- 



gressive differentiation is traceable. The chief characteristic 

 by which we distinguish dead matter is its inertness: some 

 form of independent motion is our most familiar proof of 

 life. Passing over the indefinite border-land between the 

 animal and vegetal kingdoms, we may roughly class plants 

 as organisms which, while they exhibit that kind of motion 

 implied in growth, are not only devoid of locomotive power, 

 but with some unimportant exceptions are devoid of the 

 power of moving their parts in relation to each other; and 

 thus are less differentiated from the inorganic world than 

 animals. Though in those microscopic Protophyta and Pro- 

 tozoa inhabiting the water we see locomotion produced by 

 ciliary action; yet this locomotion, while rapid relatively to 

 the sizes of their bodies, is absolutely slow. Of the Coelen- 

 terata a great part are either permanently rooted or habitu- 

 ally 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 serves as a sample, have 

 mostly but little ability to move themselves through the 

 water. Among the higher aquatic 7nvere&raa,^-cuttlefishes 

 and lobsters, for instance, there is a very considerable power 

 of locomotion; and the aquatic Vertebrata are, considered 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 characteristics of 

 self -mobility manifested in the highest degree. Flying insects, 

 mammals, birds, travel with velocities far exceeding those 

 attained by any of the lower classes of animals. 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 



