FLIGHT OF INSECTS. 253 



of the materials increases more rapidly than the strength. 

 How often has it been found that a machine which works 

 admirably in a small model, will totally fail in its perform- 

 ance when constructed on a larger scale? x\ny lever, of 

 whatever form, may be increased in its dimensions until the 

 force of gravity becomes superior to the cohesion of its own 

 particles: and consequently any structure, like a vegetable 

 or animal body, composed of a combination of levers, would, 

 if its size were to exceed a certain limit, fall to pieces mere- 

 ly by its own weight. This can be prevented either by em- 

 ploying materials of greater cohesive strength, or by in- 

 creasing, at the points where the strains are greatest, the thick- 

 ness of the parts compared with their length: but the choice 

 of materials is necessarily restricted within narrow limits, 

 and the latter expedient would entirely alter the relative 

 proportions of the parts, and would require a complete 

 change in the plan of their construction. In passing from 

 the smaller to the larger animals, we find, accordingly, that 

 new models are adopted, a new order of architecture intro- 

 duced, and new laws of development observed. "We have 

 next, then, to direct our attention to the procedure of na- 

 ture in the execution of this more enlarged and comprehen- 

 sive scheme of animal organization. 



