[June 
66 Psyche 
beginning abont 550 million years ago and extending up 
to some 190 million years ago. For convenience, it is 
divided into six periods of unequal duration, their limits 
being arbitrarily set with reference to extensive geologic 
events. In this account of early insect life, I am refer- 
ring only to the insects of the Paleozoic era — from the 
first appearance of the group through the Permian 
Period. 
The question of the oldest geologic occurrence of any 
group of organisms is always a controversial one, for 
much depends on interpretation of fragmentary material 
TABLE OF GEOLOGIC PERIODS 
ERA 
PERIOD 
approximate time 
(IN MILLIONS OF YEARS) 
DURATION 
OF PERIOD 
SINCE BEGINNING 
OF PERIOD 
Cenozoic 
(age of mammals and man) 
Quaternary 
1 
1 
Tertiary 
69 
70 
Mesozoic 
(age of reptiles) 
Cretaceous 
50 
120 
Jurassic 
35 
155 
Triassic 
35 
190 
Paleozoic 
(age of invertebrates and 
PRIMITIVE VERTEBRATES) 
Permian 
25 
215 
CARBONIFEROUS' 
Upper 
Lower 
35 
50 
250 
300 
Devonian 
50 
350 
Silurian 
40 
390 
Ordovician 
90 
480 
Cambrian 
70 
550 
Figure 1. Table of Geologic Periods. (Adapted from Romer’s Verte- 
brate Paleontology, University of Chicago Press.) 
and the definition of the group. Handlirsch showed in 
1906 that the fossils described as insects from strata older 
than those of the Upper Carboniferous Period were not 
insects at all; and he concluded that the earliest record 
of the class was in the lower part of that period. During 
the past forty years discovery of three older insects has 
been announced. Two of these, identified as Collembola, 
have been described from Devonian rocks, the Khynie 
Chert, of Scotland. 1 Without going into details, we can 
1 R. J. Tillyard, Trans. Ent. Soc. London, 1928: 65-71; H. Womersley, 
Victorian Naturalist, 1934, vol. 51 : 159—165 ; D. J. Scourfield, Proc. Linn. 
Soc. London, 152 sess., 1939-40 : 113-131. 
