44 John Henry Comstock 



as this single character is concerned a similar degree of diver- 

 gence in a similar direction may be possessed by members of 

 widely separated divisions of the order. 



But we are not entirely dependent on differences in kind of 

 specialization for indications of dichotomous divisions. Such 

 divisions may be indicated by differences in the order in which 

 specializations take place. 



This also can be illustrated by a study of the anal areas of 

 the wings. It is evident that in the primitive Lepidoptera the 

 fore wings as well as the hind wings possessed three anal veins. 

 And in certain divisions of the order the direction of special- 

 ization of the anal area of the fore wings has also been to- 

 wards a reduction in the number of veins. It will be shown 

 in the concluding part of this essay that in certain divisions of 

 the ordel" the reduction of the anal area of the hind wings has 

 preceded the reduction of the anal area of the fore wings ; 

 while in other divisions of the order the reverse is the case. 

 Here is an indication of a dichotomus division. Take for 

 example two families of moths, one of which is characterized 

 by the presence of two anal veins in the fore wings and three 

 anal veins in the hind wings ; and the other, by three anal 

 veins in the fore wings, and two in the hind wings. In the 

 former, the specialization by reduction of the anal areas has be- 

 gun in the fore wings ; in the latter, this specialization has be- 

 gun in the hind wings. And it is evident that the common 

 progenitor of the two families had three anal veins in both fore 

 and hind wings, and that the difference in the order in which 

 the reduction of the anal areas has begun indicates a dichoto- 

 mous division. 



There will also arise, I believe, in a work of this kind a ne- 

 cessity for distinguishing between the essential characters of 

 a group and those characters which are used by the systema- 

 tist merely to enable students to recognize members of the 

 group. For it seems to me that the essential characters of a 

 group of organisms do not lie necessarily in the presence or 

 absence of any structure or structures, or in the form of any 

 part or parts of the body of the living members of the group ; 

 but rather in the characteristic structure of the progenitor of 



