uOGG: TAXOXO.MIC VALUK OF SCALIvS ol' LI'. 



67 



FiR-. 13. 



Sc:ik's from siir;-le forewinii; of Heliconii sp. 



forms of the scales on ITelionia, found on the apparently clear 

 areas of the wings, arc most likely results of a retrogression from 

 specialized forms, the full course, or even true course, of ihe line of 

 specialization may not be exhibited. The series illustrating this line, 

 represented in figure 13, was taken from the forewing of one indixidual 

 of Hcliconia sp. 



In attempting to understand the significance of these different 

 modes of develoi)ment of scales, these various paths leading from the 

 generalized scale-hair to the specialized scale, it will be necessary to 

 have in mind the distinction made by Professor Comstock in his 

 essav previously referred to, between two kinds of characters which 

 will be met with by the student in trying to work out the phylogeny 

 of a grouj) of organisms, viz.: "First, characters indicating differences 

 in kind of specialization; and second, characters indicating differ- 

 ences in degree of specialization of the same kind. The former will 

 indicate dichotomous divisions of lines of descent; the latter will 

 merely indicate degrees of divergence from a primitive type." 



Looking now at the development of the scales from this point of 

 view, the gradatory series of scales taken from the Megalopygidae 

 and the series from the Cossidae represent two different kinds of 

 specialization, i. e., one a kind in which tlie change from generalized 

 scale-hair to specialized scale comes about by a splitting of the distal 

 ])ortion of the hair into two or more fingers, then a palmation or 

 filling in between the fingers until the extreme of specialization is 

 reached in a shortened, flat scale witli points or teeth projecting from 

 the outer margin; the oiher kind in which the change comes about 

 by a shortening and widening of the scale hair until there is finally a 

 short, flat, wide scale with entire outer margin. The most specialized 

 forms in each series are somewhat alike, and each has diverged from 

 the primitive type in about the same dc<^ree. The characters indicat- 

 ing the different paths taken in reaching the specialized form are 

 characters indicating dichotomous divisions of lines of development; 



