174 THE INDUCTIONS OF BIOLOGY. 



to a pound of water. The higher aquatic plants, in common 

 with the higher aquatic animals, possessing as they do in- 

 creased tenacity of substance, also contain a greater propor- 

 tion of the organic elements ; further they show us a greater 

 variety of composition in their different parts; and thus in 

 both ways are chemically more unlike their medium. And 

 when we pass to the superior classes of organisms land- 

 plants and land-animals we see that, chemically considered, 

 they have little in common either with the earth on which 

 they stand or the air which surrounds them. In 



specific gravity too, we may note a like truth. The simplest 

 forms, in common with the spores and gemmules of higher 

 ones, are as nearly as may be of the same specific gravity as 

 the water in which they float; and though it cannot be said 

 that among aquatic creatures, superior specific gravity is a 

 standard of general superiority, yet we may fairly say that 

 the higher orders of them, ivhen divested of the appliances 

 by which their specific gravity is regulated, differ more from 

 water in their relative weights than do the lowest. In ter- 

 restrial organisms, the contrast becomes marked. Trees and 

 plants, in 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 fulfilled in respect of temperature. 

 Plants generate but extremely small quantities of heat, which 

 are to be detected only by delicate experiments; and prac- 

 tically they may be considered as having the same tempera- 

 ture 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 

 fishes, 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 



