1930 ] 
Geological History of Insects 
25 
Thysanoptera to the Triassic, while the Lepidoptera and 
Isoptera would be shown as far back as the Jurassic only. 
Whether these estimations are substantiated or not will of 
course depend entirely upon the future discoveries in insect 
paleontology. 
There is one other aspect of insect paleontology which I 
wish to discuss : the relative development of each order in 
the several geological periods. Those of you who have 
never considered this aspect of entomology will probably be 
more or less surprised at some of the facts. The average 
entomologist, if there be such a freak, is so accustomed to 
thinking of the insect orders in their present relative stand- 
ings that he never stops to reflect that there must have been 
some period in the earth’s history when the more predomi- 
nant of the recent orders were actually struggling for a 
footing; when some of our smaller groups, now nearly ex- 
tinct, were the predominant ones. Or perhaps the entomol- 
ogist is, let us say, a hymenopterist, and so fond of his pets 
that he cannot imagine any period in the earth’s history 
when they were not disconcertingly abundant. But a mo- 
ment’s reflection on the geological history of the other 
groups of animals will show that such a change in the rela- 
tive status of the orders is only to be expected. Taking the 
fishes for an example, we note that the Lung Fishes, which 
are now represented by a very few species, were one of the 
most predominant groups during the Devonian and Carbon- 
iferous; in a similar manner the Lobe Fishes and the 
sharks were very abundant during the latter half of the 
Paleozoic, although both of these types are now greatly 
outnumbered by more recently evolved forms. Innumerable 
examples may also be found in the higher vertebrate 
classes. Among the Reptilia, the Order Crocodilia was rep- 
resented in the middle Mesozoic by a great number of spe- 
cies, which are now reduced to a small fraction. An even 
more striking illustration is furnished by the Rhyncoce- 
phalia, which were well developed in the number of species 
during the Triassic, but are now known from a single liv- 
ing species, Sphenodon punctata, of New Zealand. It is only 
logical, therefore, that we should find that our insect orders 
have passed thru similar modifications. 
