LIFE ON THE EAKTH. 39 



truths of mechanical science, nor could the human 

 mind with the materials given have predicted by any 

 theory the arrangement which is found to be so com- 

 plete. Into the other wonders of the feather, its 

 hollow quill basis, its implantation, the relation of 

 the directions of its axis and plane to the axes and 

 planes of the plumose elements I cannot now enter, 

 though by this means the impression that the whole 

 is an ingenious and profound piece of mechanism 

 would surely be strengthened. 



The scheme of feather structure and arrange- 

 ment is altered in the tails of Birds to suit the very 

 different mechanical purposes of that instrument 

 altered in the construction of the individual feathers 

 in the direction of their planes in the resultant 

 of their strength. Hence the resemblance of the 

 steering-tails of the swift Falconidae, and Hirundines, 

 and Sternidse in contrast with the stiff prop of the 

 Picidae, and the almost extinction of the apparatus 

 and suppression of the function in the acuminated 

 tails of the Diving-birds. Every feather is altered 

 when the work is different. 



Among Insects the Lepidoptera, with their ex- 

 panded wings, shew the same principle, differently 

 carried out; what in human contrivances is called 

 1 feathering the oar' being largely employed in na- 

 ture; and in Coleoptera we may cite the curious 



