LEPIDOPTEROCS WINGS. 35 



is always such as to give the greatest possible degree 

 both of strength and of stiffness with the least pos- 

 sible quantity of materials. 



The distinctive characters which are traceable in 

 the covering and contents of the egg, are found in 

 an especially conspicuous manner in the covering of 

 the bird. All birds are covered with feathers ; and 

 they are the only animals which, properly speaking, 

 are so. These feathers are of two sorts, feathers 

 for clothing, to protect the animal from the vicissitudes 

 of the weather, and feathers for flight. Both of these 

 are beautifully modified so as to suit the different 

 habits of the several species, and adapt them to the 

 climates and the elements in which they find their 

 food. 



Some other animals, as for instance the lepidop- 

 terous insects the butterflies and the moths have a 

 coat of feathers, or rather of fringed or feathery 

 scales ; but these have few or none of the characters 

 of true feathers, and in no case, except that of birds, 

 are feathers the instruments of flight. But still we 

 can, in the imperfect feathers of the lepidoptera, dis- 

 cover one of the uses of feathers in birds better than 

 we can perhaps do in the feathers of birds them- 

 selves, as in them it is conjoined with other uses. 

 The study of one animal often assists us in acquir- 

 ing a knowledge of another, especially when the 

 one contains a single part of that which is a com- 

 pound organ in another ; because by this means we 

 get an analysis of the living animal, which is far more 

 satisfactory than any that we could obtain by the 

 dissection of a dead one ; for we can, in the one 

 case, actually see the part of the organ in action, 

 whereas in the other we can only infer or guess at 

 the way in which it acts. 



