1930] 
Geological History of Insects 
17 
insects, as well as those of the Tertiary of England and 
others contained in the Burmese amber. ' Very recently 
Martynov has described the insect fauna of a newly dis- 
covered Tertiary bed in Russia, and Pongraz has written 
several papers on the fossil insects of Hungary, redescrib- 
ing many obscure insects which Heer first studied seventy 
years ago. A few Tertiary insects have been collected at 
new localities in this hemisphere, — in Tennessee, Washing- 
ton, Nevada, and Argentina, all of which give much promise 
of providing us with a more complete series of fossils in 
the future. 
If we take the conventional bird’s-eye view of the geolog- 
ical history of the insects, we are at once struck by the 
antiquity, not only of their natural groups, such as the 
orders, families, and genera, but of their habits and ethol- 
ogy. As far back as the Oligocene, about 50 million years 
ago, the social Hymenoptera had already differentiated into 
several castes, and the ants, at least, had accomplished this 
by the Middle Eocene. The Baltic amber ants, as demon- 
strated by Wheeler, show definite polymorphism and even 
the higher stages of development such as ergatoid and pseu- 
dogynic females, and ergatomorphic males. Some of these 
Oligocene ants had also “learned to attend” plant-lice, just 
as many of the modern species do. Whether or not some of 
the amber insects belong to species which are still existing 
on earth, is an open question. In the case of the ants, there 
are eight species which are morphologically identical with 
certain living ones. If these species actually are identical 
— and there is no evidence to the contrary — then they have 
existed without apparent structural change for some 50 
million years 4 . Aside from the probability of the specific 
identity of some of our fossil and recent insects, it is cer- 
tain that most of the genera of the Tertiary are still sur- 
viving. Of course many of the genera which used to be more 
or less cosmopolitan, these many millions of years ago, are 
now restricted to much smaller areas, — as the dipterous 
genus, Glossina or the formicid, Oecophylla. As we be- 
come more and more acquainted with the tropical insect 
4 I have used the time estimates advanced by Dr. A. Holmes, in his 
“Age of the Earth” (London, 1927). 
