44 
Psyche 
[ March 
cold climates. Among reptiles the largest forms are very 
clearly those of the tropics. Without multiplying examples 
further it is evident that neither warm nor cold climates 
can be regarded as constant stimuli to large size among 
animals in general. Thus certain types appear to wax 
larger in one to which they are usually said to be best 
adapted. The accuracy of such a conclusion is certainly 
open to question, as it is based on the dubious assumption 
that increased size is indicative of a more perfect adapta- 
tion to the environment. 
I shall hope to be pardoned for introducing the foregoing 
long series of commonplace statements, but it appears nec- 
essary to have them in mind before going any further. 
In general, the average size of insects has probably 
decreased during the history of the group on the earth, at 
least during its early history, but these changes have been 
very slight compared with the great increases in size that 
occurred, for example, very early in the history of the mam- 
malia, in certain special groups of mammals, and among 
the mesozoic reptiles during their period of ascendency. 
Among these mammals and reptiles this great increase in 
size has consistently led to the extinction of the groups in 
which it occurred. Since some primitive types of living in- 
sects are of very small size, we may reasonably believe that 
such small forms were more abundant during the early 
history of insects than actual paleontological discoveries 
have yet been able to show. We know in some cases like 
that of the early giant dragon-flies that comparatively large 
size still persists as a general characteristic of recent 
Odonata. The same is true generally in other groups like 
the Blattariae, Orthoptera, Mantodea and Corrodentia which 
tend to adhere rather closely to the size-range which pre- 
vailed among their ancient prototypes. Thus conservatism 
in size appears to be a characteristic of insect groups over 
long periods of time. The failure of any insects to attain 
a size comparable to that of certain giant Crustacea is 
usually attributed to their aerial habitat which renders 
their soft bodies highly susceptible to gravitational and 
traumatic deformation at the time of molting. This is 
certainly not the entire story, however, as there are no un- 
