8 University of California Publications. [Entomology 



just made. The paleontological evidence is all to the effect that 

 no true elytriferous insects occurred till after the Paleozoic 

 era. The cockroaches of the oldest strata, for instance, still 

 possess comparatively membranous fore-wings. 



Miiller's theory thus seems not to have been particularly 

 strengthened by these contributions, and really rests on the 

 original ontogenetic observations. Unless other facts can be 

 discovered that will give undoubted support to this theory, it 

 certainly has no sound foundation, for the well-known tendency 

 toward short cuts in ontogeny will easily account for the facts 

 observed by Miiller in regard to the tracheation of the young 

 wingpads in Caloterrnes. 



Gegenbaur's theory stands, therefore, not only as the gen- 

 erally accepted one, but as a theory against which no very 

 significant objection has been raised, nor has it an important 

 rival. There are grounds, however, more important than any 

 that have been raised, for objecting to this theory in its original 

 and usually accepted form. It may safely be said that the 

 histological conditions essential to the function of blood aera- 

 tion are far different from such as would be necessary in the 

 construction of an organ of flight. A wing could not have 

 been directly produced from a tracheal gill. The changes that 

 must occur in the production of a wing are so great as to 

 require, as an intermediate stage, the modification of the gill 

 into an organ with a different function. 



When an organ changes from one structure and function 

 to another it must be possible for the organ to serve for a 

 while the former function, but there must also be requirements 

 in the first function that will cause a development of the 

 organ to a stage where it can serve also in the new function. 



In no tracheal gill not specialized somewhat for some other 

 than the primary function is there a sufficient approach either 

 in size, form, or structure toAvard an organ that could assume 

 in the slightest degree the function of flight. It seems neces- 

 sary for this reason to modify Gegenbaur's theory, as has been 

 done by Lang ('88), so as to recognize the tracheal gill as one 

 of the two necessary steps before the production of a wing is a 

 possibility.. 



No one has yet attempted to consider the detail of wing 

 specialization, or to inquire into the process or method by which 

 it is brought about. A complete explanation requires that the 



