1922] Crampton — Relationship of Hemiptera-Homoptera 37 



chanced to use a fore wing of an insect belonging to the genus 

 Psijlla to iUustrate the operation of the same developmental 

 tendencies in the evolution of the wing veins throughout the 

 orders Homoptera and Psocida. The wing type exhibited by 

 Psylla, however, is but one of a wide-ranging series of forms. 

 (a few of which are shown in Figs. 17 to 24), extending from the 

 lower Psocids and Homoptera to the higher speciahzed members 

 of the two groups, in which the developmental tendencies 

 operative in directing the evolution of the various types of 

 venation in the Psocid wings are closely paralleled throughout 

 the series by similar developmental tendencies operating in the 

 evolution of the various types of Homopterous wings. In other 

 words, the same genes, determinants or factors were in many- 

 cases inherited in both groups from a common ancestry, although 

 they were naturally modified somewhat by different factors in 

 the two distinct orders of insects. This again is a very different 

 matter from claiming that all Homoptera were descended from 

 the highly specialized recent Homopterous family Psyllidae, 

 and I am at a loss to understand how Mr. Muir could have sO' 

 completely misconstrued my meaning in this matter. 



As a final and culminating false assumption, Mr. Muir implies 

 that I "believe that new orders arise as hybrids from the crossing 

 of individuals belonging to different orders" of insects! The 

 fact that every student of evolution knows full well that the off- 

 spring of crosses between different species are generally sterile, 

 and those between different genera are almost invariably so 

 (save in the plant kingdom) should have deterred Mr. Muir 

 from making this curious mistake. However, lest others be 

 misled by Mr. Muir's implication, I would endeavor to indicate 

 graphically by means the diagram shown in Text figure 2, how 

 a third order of insects may partake of characters present in two 

 other orders, without being the result of the crossing of members 

 of the other two orders possessing characters in common with it. 

 I have drawn a similar diagram, and explained it, in an article 

 published in the Fiftieth Annual Report of the Ent. Society of 

 Ontario for 1919; and in order to use the same concrete examples, 

 let us suppose by way of illustration that "A" in Text figure 2 



