354 THE MECHANICAL FUNCTIONS. 



prevails universally in the animal kingdom, and 

 modifies the structure of each individual part 

 so as to preserve its conformity to one general 

 type. 



The innumerable tribes of butterflies, sphinxes, 

 and moths, are all comprehended in the order 

 Lepidoptera, and are distinguished by having 

 wings covered with minute plumes, or scales. 

 These scales are attached so slightly to the 

 membrane of the wing as to come off when 

 touched with the fingers, to which they adhere 

 like fine dust. When examined with the mi- 

 croscope, their construction and arrangement 

 appear to be exceedingly beautiful, being 

 marked with parallel and equidistant striae, 

 often crossed by still finer lines, the distinct 

 visibility of which in many kinds of scales, as 

 those of Pontia brassica, or cabbage butterfly, 

 and the 3Iorpho 3Ieuelaus of America, consti- 

 tutes a good criterion of the excellence of the 

 instrument. The beautiful colours which these 

 scales possess may perhaps generally be owing 

 to the presence of some colouring material : but 

 the more delicate hues are probably the result 

 of the optical effect of the striae on the surface ; 

 and in some cases they result from the thin- 

 ness of the transparent plate of which they con- 

 sist ; for I have observed in several detached 

 scales that the colours they exhibit by trans- 

 mitted light are the complementary colours to 



