14:6 THE INDVCIIONS OF BIOLOGY. 



creatures, superior specific gravity is a standard of general 

 superiority, yet we may fairly say that the superior orders of 

 fchem, when divested of the appliances by which their specific 

 gravity is regulated, differ more from water in their relative 

 weight than do the lowest. In terrestrial organisms, the 

 Contrast becomes extremely marked. Trees and plants, :n 

 common with insects, reptiles, mammals, birds, are all of a 

 specific gravity considerably less than that of the earth and 

 immensely greater than that of the air. Yet further, 



we see the law similarly fulfilled in respect of temperature. 

 Plants generate but extremely small quantities of heat, which 

 are to be detected only by very delicate experiments ; and 

 practicalh^ they may be considered as having the same tem- 

 perature as their environment. The temperature of aquatic 

 animals is very little above that of the surrounding water : 

 that of the invertebrata being mostly less than a degree above 

 it, and that of fishes not exceeding it by more than two or 

 three degrees ; save in the case of some large red-blooded 

 fi-shes, as the tunny, which exceed it in temperature by nearly 

 ten degrees. Among insects, the range is from two to ten 

 degrees above that of the air : the excess varying according 

 to their activity. The heat of reptiles is from four to fifteen 

 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-mohility a pro- 



gressive differentiation is traceable. The especial character- 

 istic by which we distinguish dead matter is its inertness : 

 some form of independent motion is our most general test 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 species 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 



