BY R. J. TILLYARD. 539 



Fossil types must be carefully compared with recent types show- 

 ing closely similar venations; but wide generalisations, based for 

 the most part upon the study of many types lying outside the 

 bounds of the Complex, must be avoided, as tending to give 

 very misleading results. As an example of this, we may cite 

 Comstoek's insistence on a single primitive venational type (15 

 Chap. iv. ) . which he constructed largely with the aid of the 

 Carboniferous Palaeodietyoptera, and then applied to the Lepid- 

 optera and other Orders within the Panorpoid Complex. The 

 primitive type to be considered in this paper will not be the 

 one constructed by Comstock, but one constructed on the evidence 

 obtainable within the Complex only; and it will be found to 

 differ in certain very important respects from that type which, 

 whether it was the primitive type for the Pterygote Insects or 

 not, was certainly not, in some of its essential characters, the 

 primitive type from which the Panorpoid Orders were derived. 



As an instance of the value of the pala?ontological data, let 

 anyone after reading this paper, cut right out of it all the evi- 

 dence afforded by the three fossil Orders Protomecoptera, Para- 

 mecoptera and Paratrichoptera, and then consider what chance 

 there would have been of a correct conclusion on the remaining 

 evidence alone ! Let him also consider what the result might 

 have been, if not only this evidence, but also that afforded by 

 the fossil representatives of still existing Orders, had not been 

 available? It is not always difficult to construct a simple theory 

 that will fit all the known facts; the chances that Nature, in the 

 course of Evolution, followed the direct path that such a theory 

 is sure to lay down, are actually very small indeed. 



(D) Convex and Concave Veins. It is easy to demonstrate 

 that the more archaic types within every Order of the Panorpoid 

 Complex show the same arrangement of Convex and Concave 



R Cu 

 Ct. ^ ^ 



Text-fig. 35. 

 Two sections across a wing of Panorpoid type, to show alternation of 

 convex and concave veins, a, close to base; b, beyond r/"and mf. 

 Lettering as on p. 535. (Cf. Text-fig-16, corrected, on p. 713). 



