322 THE MF.CHANICAL FUNCTIONS. 



creased in its dimensions until the force of gravity 

 becomes superior to the cohesion of its own par- 

 ticles ; and consequently any structure, like a 

 vegetable or animal body, composed of a combina- 

 tion of levers, would, if its size were to exceed a 

 certain limit, fall to pieces merely by its own 

 weight. This can be prevented either by employ- 

 ing materials of greater cohesive strength, or by 

 increasing, at the points where the strains are 

 greatest, the thickness of the parts compared with 

 their length : but the choice of materials is neces- 

 sarily restricted within narrow limits, and the latter 

 expedient would entirely alter the relative propor- 

 tions of the parts, and would require a complete 

 change in the plan of their construction. In pass- 

 ing from the smaller to the larger animals, we find, 

 accordingly, that new models are adopted, a new 

 order of architecture introduced, and new laws of 

 developement observed. We have, next, then, to 

 direct our attention to the procedure of nature in 

 the execution of this more enlarged and compre- 

 hensive scheme of animal organization. 



Chapter VI. 



VERTEBRATA. 



^ 1. Vertehrated Animals in geiieral. 



If it be pleasing to trace the footsteps of nature in 

 constructions so infinitely varied as those of the 

 lower animals, and to follow the gradations of 

 ascent from the zoophyte to the winged insect, 



