6 
Psyche 
[March 
through them to other animals, coupled with their great 
abundance and diversity, they are of extreme interest. Their 
ecological relations are far reaching for they extend, ten- 
tacle-like, into the innermost corners of every type of terres- 
trial, aerial, or fresh-water environment. From the general 
biological or evolutionary standpoint they are of peculiar 
interest for it is this vast horde of vegetarian species (for 
they include about half of the living kinds of insects) that 
have made their influence felt over the long lapse of geo- 
logical time since these types became highly differentiated 
during the periods preceding the tertiary. The chronology 
of this process, at least with regard to specific food rela- 
tions, is difficult to determine, but taxonomic groups similar 
to those of today were so well established in the eocene and 
oligocene that we can rest assured that their food relations 
were already equally complex at that time. Thus, the time 
during which the factors introduced by these insects have 
been active in affecting the evolution of other animals and 
plants is much more extended than that included in the 
period just mentioned. As I have shown previously, there 
is good reason to believe that the differentiation of feeding 
habits among phytophagous Coleoptera was well under way 
while that of the Lepidoptera was just beginning at the 
time when the modern types of trees appeared on the earth. 
There is no need to attempt at the present time any de- 
tailed account of the specificity of food selection among the 
Lepidoptera as this is well known and I have already dealt 
extensively with it elsewhere. Briefly, we may say that the 
members of this order may be considered as forming two 
or three groups with respect to specificity of food. These 
are: first, those which utilize a very considerable and not 
necessarily related series of food plants, occasionally a hun- 
dred or more in number, like the cecropia moth or the leop- 
ard moth; second, those having a much more restricted 
dietary that includes a few, usually related, species; and 
finally, some that are confined to a single plant host or to 
several very closely related and genetically similar members 
of a single genus. Again, these categorical divisions are 
only relative; but experience shows them to be very con- 
venient, and we may unquestionably regard them as suc- 
cessive phylogenetic stages. We may conveniently refer to 
