58 John Henry Comstock 



Although the wings of the Palaeodictyoptera agree in the 

 characteristics given above they present great differences of 

 structure. How shall we decide which of the different forms 

 is the more primitive. This I think can he done by determin- 

 ing the degree of their adaptation to the performance of their 

 function. 



In those insects which have the best developed powers of 

 flight we find that the costal edge of the front wings is the 

 strongest part of the wing, the strength being due to the 

 presence of several strong veins which are nearly parallel. 

 Such an arrangement is necessary to withstand the strain that 

 is brought upon this part of the wing. I conclude, therefore, 

 that wings possessing this structure are more highly special- 

 ized than those in which the costal edge is not strengthened 

 in this way. Using this criterion I select that form of wing 

 which departs most widely from this type as the most gener- 

 alized form ; for so important a character as this when once 

 attained would not be lost so long as the wings were used as 

 organs of vigorous flight. I conclude, therefore, that it is 

 among the carboniferous coachroaches that we find the most 

 generalized form of wing. 



Owing to our limited knowledge of extinct forms, I cannot 

 hope to present in these studies continuous series ; but can only 

 select examples which illustrate the direction or directions of 

 specialization of particular parts, without strictly confining 

 myself to a single line of development. 



In Mylactis anthracophilum (Fig. 7) the wing is furnished 



with five sets of veins in addition to the costal or marginal 



vein if this exists. Three of these which oc- 



cup3' the intermediate area of the wing arise 



each in a single strong trunk. These veins 



__,..,^_^ may be designated beainnino- with the one 



nearest the costal margin as the i-adiiis, media. 

 Fig. j.—Myla- , ; •, .• ^ 



cris anthraco- ^nd ciibitus, respectively. 



philum. Lying between the costal edge of the wing 



and the radius there is a group of veins which 



radiate from near the base of the wing (Fig. 7, II). These I 



believe represent the subcosta in its most generalized form. 



