1930 ] 
Geological History of Insects 
31 
Permian. It is obvious, I think, that the jump from 1% in 
the Permian to 37 % in the Mesozoic, is so great as to arouse 
one’s suspicions as to the accuracy of the geological records 
of these insects. For my own part, I suspect that the fault 
lies with the Permian, not with the Mesozoic, and that a 
relatively larger number will be found in the Permian than 
we know at present ; but it seems unlikely that the Permian 
ratio will ever approach that of the Mesozoic. The Diptera 
have likewise been constant in their occurrence in insect 
beds. It certainly does not seem logical that the Tertiary 
proportions, somewhere around 25% will ever be exceeded 
by those of the Mesozoic, which have not gone over 5%. 
Whether the Diptera were actually twice as abundant rela- 
tively in the Tertiary as they are at present, as our figures 
would indicate, is perhaps open to more question; there is 
certainly no reason why this order should not have attained 
its maximum during the Tertiary. When we pass to the 
Plectoptera or Ephemerids, we again find in the Mesozoic 
and Tertiary a stable list of percentages. The Tertiary fig- 
ures are much lower than those of the Mesozoic, and would, 
in fact, require an increase of 600% to bring them to the 
same magnitude. The Permian percentage in the 1930 
column are not quite twice those of the Mesozoic, so that it 
is perfectly possible that sometime we may have sufficient 
records to show that the may-flies were relatively more 
abundant in the Mesozoic than in the Permian. From the 
standpoint of comparative morphology, however, this is 
unlikely, for these insects are generally recognized as being 
the most primitive of any insects now existing. The 
Mesozoic and Tertiary records of the Blattids are also very 
constant, and since that of the Mesozoic is far ahead of the 
one in the Tertiary, we certainly cannot look for a reversal 
of the present ratios. The figures of all the geological pe- 
riods point definitely to the conclusion that the cockroaches 
reached their highest development in the number of species 
during the Upper Carboniferous, and have been decreasing 
right down to the present time. Of all our records that of 
the cockroaches is the least open to radical change. The 
last order on our list, the Orthoptera, has turned up rather 
regularly in the various geological formations, and the per- 
centage of the Mesozoic species is so far ahead of either 
