ON DESIGN IN ANIMAL BODIES. 209 



ternal covering instead of bones, have compara- 

 tively much less material in them than the larger. 

 Accordingly, philosophers have contrasted the 

 power of the flea w^ith that of the horse, deciding 

 greatly in favour of the former. The rationale of 

 this is not quite apparent at first ; but a little con- 

 sideration will convince us, that the resisting ma- 

 terial being exterior to the animal's body, and con- 

 sequently removed from the centre, it must pos- 

 sess more power against transverse fracture, as 

 well as bestow a mechanical advantage for the 

 action of the muscles. But this is not all : any de- 

 gree of density and strength may be given to it, 

 from its being a mere secretion, and being unor- 

 ganized. We may compare, however, the bones 

 of man with those of the elephant, or other huge 

 animals.* Now, it would seem that the material 

 of bone (which we must recollect is porous, since 

 it constitutes a living part, and is nourished by 

 blood-vessels) could not, by any variety of confor- 

 mation, bear up a greater mass than that of the 

 elephant. On examining the bones of these im- 

 mense animals, including the megatherium and 

 rhinoceros, they are dense and strong, and clumsy, 

 as we would term it ; their spines and processes 

 are large, and their cavities filled up : all which 

 indicates, that to support a larger animal on ex- 

 tremities, some other material than the vascular 

 bone would be required. Those immense bones 



* Vol. i. p. 309. 

 19* 



