164 Dr. T. A. Chapman on the 



There can, it seems, be no doubt that the membranous 

 sheath of the aedeagus is a portion of the external surface, 

 probably of the 10th abdominal segment, and still more 

 certainly that the free (external) portion of the aedeagus 

 is of internal origin, i. e. a portion of the azygos duct. 

 The internal portion, consisting of two fused layers, may 

 present material for doubt, but the doubt has no particular 

 bearing on the subject of this communication, though of 

 morphological interest and suggesting further investigations . 



For my own part I incline to the opinion that the zone 

 represents the circle of the original opening of the ductus 

 on the surface, and that the aedeagus consists of reflections 

 and invaginations of the ductus only, without any surface 

 structures being incorporated. 



The period before the evolution of the aedeagus, when 

 the ductus opened simply on the surface must be very 

 remote, certainly before the Lepidoptera originated, 

 possibly antecedent to the Insecta. 



Whenever it may have been, the date was so ancient 

 that one is astonished to find the aedeagus and ductus 

 still so plastic, and capable of so rapidly (as between two 

 closely related species) changing their form. No doubt 

 the selection that evolved species also insisted throughout 

 on the plasticity of these structures, without which new 

 species would frequently have failed to establish them- 

 selves ; since a fixity of structure, such as is usual in most 

 other portions of the organism, would easily have led to 

 syngamic absorption of the new species before its segrega- 

 tion was sufficiently prolonged to give real specific separa- 

 tion. In other words, a species in which these organs 

 refused to vary, would on meeting with an incipient 

 species derived from it simply absorb it, i. e. it could 

 not meet new conditions by offsets of new species, and 

 could only vary en masse, and would be very likely to 

 become extinct. The incipient species could not avoid 

 absorption unless its segregation (in whatever way) had 

 been sufficiently prolonged to make it no longer incipient. 



It is difficult to suppose, in the case of such groups as 

 our Plebeiidi, that segregation could often have been so 

 prolonged, so many of the species having such similar 

 habits and habitats. 



In the Plebeiids, the relation of the aedeagus to the 

 genital cavity is reduced to its simplest elements. The 

 floor of the cavity is a simple smooth screen, reaching from 



