1922 ] Crampton — Relationship of Hemiptera-Homoptera 
37 
chanced to use a fore wing of an insect belonging to the genus 
Psylla to illustrate 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 specialized 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 Psyllidse, 
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 m lext figuic — 
