Evolution and Taxo7iomv 



11 



Not only niai' the branches of one system of veins become 

 joined to those of other systems as just described, but there 

 are many forms in which two adjacent principal veins are 

 coalesced to a greater or less extent. This occurs chiefly in 

 the hind wings. 



I will discuss the veins in regular order, beginning with 

 costa. This vein is apparently wanting in the hind wings of 

 most Lepidoptera , 

 and but little can be 

 said regarding the 

 manner of its disap- 

 pearance. It .seems 

 probable that in most 

 cases it has simply 

 become atrophied, 

 the overlapping of 

 the wings rendering 

 it unnecessary or even 

 undesirable. For 

 when that stage in 

 the development of 

 the order was reached 

 in which the two hind 

 wings of each side 

 overlapped to a con- 

 siderable extent, was it not better that the costal margin 

 of the hind wing should be flexible ? There was no longer 

 any need of a stiff margin, this part of the wing being sup- 

 ported during the downward stroke by the overlapping part 

 of the fore wing ; while a flexible margin would act as a 

 valve to prevent the escape of the air between the two wings. 

 The two wings in this way present a continuous surface. In 

 many moths there is a thickening of the basal part of the 

 costal margin ; this I believe to be the remnant of costa. 



But although it seems probable that in many cases the 

 costa of the hind wings has simply faded out leaving cell I to 

 function as this costal valve, there are cases in which this 

 valve is a precostal development, the costa having moved 



ZygcFua. 



