162 Phenomena of Variation and Geographical Didrihntion. 



furnish materials sufficiently approaching to completeness to enable us 

 to arrive at any accurate conclusions as to the phenomena they pre" 

 sent as a whole. It is in those groups which are and have long been 

 favorites with collectors, that the student of distribution and variation 

 will find his materials the most* satisfactory, from their comparative 

 completeness. 



Preeminent among such groups are the diurnal Lepidoptera or but- 

 terflies, whose extreme beauty and endless diversity have led to their 

 having been assiduously collected in all parts of the world, and to the 

 )uimerous species and varieties having been figured in a series of 

 magnificent works, from those of Cramer, the contemporary of Linnasus, 

 down to the inimitable production of our own Hewitson. But, besides 

 their abundance, their universal distribution, and the great attention 

 that has been paid to them, these insects have other qualities that 

 especially adapt them to elucidate the branches of inquiry already 

 alluded to. These are the immense development and peculiar struc- 

 ture of the wings, which not only vary in form more than those of any 

 other insects, but offer on both surfaces an endless variety of pattern, 

 coloring, and texture. The scales with which they are more or less 

 completely covered, imitate the rich hues and delicate surfaces of 

 satin or of velvet, glitter with metallic lustre, or grow with the change- 

 able tints of the opal. This delicately painted surface acts as a register 

 of the minutest differences of organization — a shade of color, an 

 additional streak or spot, a slight modification of outline continually 

 recurring with the greatest regularity and fixity, while the body and 

 all its other mend^ers exhibit no appreciable change. The wings of 

 butterflies, as Mr. Bates has well put it, "serve as a tablet on which 

 Nature writes the story of the modifications of species ; " they enable 

 us to perceive changes that would otherwise be uncertain and difficult 

 of observation, and exhibit to us on an enlarged scale the effects of 

 the climatal and other physical conditions which influence, more or 

 less profoundly, the organization of every living thing. 



A proof that this greater sensibility to modifying causes is not 

 imaginary, may, I think, be drawn from the considerati(;)n that while 

 the Lepidoptera as a whole are of all insects the least essentially varied 

 in form, structure, or habits, yet in the number of their specific forms 

 they are not much inferior to those orders which range over a much 

 wider field of nature, and exhibit more deeply seated structural mod- 

 ifications. The Lepidoptera are all vegetable feeders in their larvte 

 state, and suckers of juices or other liquids in their perfect form. In 

 their most widely separated groups they differ but little from a common 

 type, and offer comparatively unimportant modifications of structure 



