32 
Psyche 
[March 
the Permian or the Tertiary that we are justified in con- 
cluding* that the order reached its maximum development 
during the Mesozoic. 
For my own part, therefore, I believe that the above per- 
centages, indicating the relative abundance of the species 
in each order during the several geological periods, is ap- 
proximately correct for all the orders mentioned, except 
probably the Neuroptera and Coleoptera. On that basis, at 
any rate, we may separate the orders into several groups, 
based upon the time of the maximum development of the 
order. The Mecoptera, Homoptera, Psocoptera, Plectop- 
tera, and Blattaria are alike in that they had reached their 
maximum by the Permian. This result is not at all surpris- 
ing when we reflect that this is precisely what we should 
expect from the morphology of these insects. For a com- 
parative study of their structure has demonstrated that 
every one of the orders mentioned is very primitive. It is 
probable, as I have noted above, that the Neuroptera belong 
to this series. The next group of orders, those which 
reached their highest development in the number of species 
during the Mesozoic, includes the Odonata and the Orthop- 
tera. Here again w T e find this situation perfectly consistent 
with the results of comparative morphologists, for these 
two orders, while primitive in many respects, are a little 
more highly specialized than those which we have just con- 
sidered. There remains, then, only a single order, the Dip- 
tera, which at present seems to have attained its peak dur- 
ing the Tertiary. And once more we are consistent in our 
conclusions with those of morphological studies, for the flies 
are more highly specialized than any of the orders included 
in the foregoing groups. In this discussion of the develop- 
ment of the insect orders, I have omitted any mention of 
the Perlaria or stone-flies, because just at present the geo- 
logical record of these insects is much confused owing to 
difficulties in interpreting the venation. I have also omit- 
ted reference to certain other groups, such as the Heterop- 
tera, Trichoptera, Lepidoptera, Isoptera, and Hymenop- 
tera, none of which has been found in rocks older than the 
Mesozoic. All these appear to be younger groups, with a 
shorter and perhaps less completely known geological his- 
tory than the ones which we have considered. Most of them 
