874 RADIAL AND ZYGOPTERID SECTORS, &C., 



homologies of the other trachese are only the result of con- 

 vergence (a position I can't admit), and Es and Ms are really 

 quite different things. I should be glad to have your explana- 

 tion of these difficulties (as they seem to me).'"' 



Now this is a very lucid statement of the case, and one with 

 which I must confess great sympathy. Like Dr. Calvert, I could 

 not for a moment admit that Anisoptera and Zvgoptera had no 

 common ancestry, though I would hold, perhaps, that the cleavage 

 between them is greater, and that the common ancestiy lies 

 further back in geological time, than Dr. Calvert may be pre- 

 pared to grant. For this reason, I felt that the researches 

 which I had carried out could not be left in their present state. 

 Having satisfied myself (hat Neosticta (the only doubtful genus) 

 falls into line with all the rest, I was spurred on, by Dr. 

 Calvert's very reasonable statement of the case, to investigate 

 the whole problem as fully as possible, so as to review the whole 

 evidence, and to come to some definite conclusion. 



In order to delimit the problem, let me state clearly at the 

 start that, as far as I can see it, we have actually two cognate 

 problems to deal with. The first of these concerns the relation- 

 ship between trachea'Rs, and trachea Ms, the second that between 

 vein Rs and vein Ms. As we shall see in the sequel, these two 

 problems may very well lead to different results. 



Leaving aside altogether the question of palteontological evi- 

 dence, which cannot be admitted into this problem satisfactorily, 

 owing to the complete absence of the tracheational interpretation 

 of fossil vein-formations, I have classed the available evidence 

 under three main headings :— 



(1) 2'he structural evidence. By this I mean the evidence 

 obtained from a study of the wing-tracheation of the last larval 

 instar, and the interpretation of the corresponding imaginal 

 wing-venation. 



(2) 7'Ae ontogenetic evideiice. This is the evidence obtained 

 from the tracheation of the wings of the developing larva, from 

 the earliest examinable stage up to the last instar. 



(3) The evidence from Bridges and Oblique Veins. The con- 

 nection of these structures with the problem in hand is not, per- 



