664 og The Rocky Mountain Locust. [ November, 
Caloptenus spretus, and is purely American, since it does not in- 
habit any other continent. 
Evolutionists believe — and I am one of them — that existing 
species are but the modified descendants of preéxisting species. 
The present species of a genus have’ at some time, more or less 
remote, had a common ancestry. All life exhibits a certain 
power of adaptation to surrounding conditions, and through what 
is known as “natural selection”? (two words which by Darwin’s 
pregnant pen have come to express volumes of facts and conse- 
quences), coupled with other less easily formularized laws, the 
fauna and flora of the globe have been as profoundly changed as 
have its physical conditions. The influences that have thus 
worked in the past are still working at present — less rapidly, 
perhaps, in the’ main, but none the less effectually. Among 
higher and more complex animals the changes are slow and not 
very noticeable ; the species have become, in most cases, markedly 
differentiated, and their characters are well fixed. Among lower 
organisms these changes are more obvious, and naturalists are 
sorely puzzled in their endeavors to grasp and express them. 
This is especially the case among insects. We have the simple 
variation from the typical characters of a species; we have phy- 
tophagic varieties, or those departures from the type that result 
from the kind of food assimilated during growth ; we have phy- 
tophagic species, or those variations which have become fixed and 
permanent in the adolescent or immature stages through some 
peculiar and fixed habit, without having yet modified the imago 
or mature state; we have geographical variation, increasing — 
„usually with distance — until the separation from the type is suf- 
ficient to be indicated by what we call race; we have seasonal 
variation, sexual variation, and, finally, we have the terms dimor- 
phism, heteromorphism, and many other isms, to express still 
other variations. In short, in the strain, the breed, the sport, the 
tribe (in the popular sense), the variety, and the race, we have 
so many terms invented to indicate some of the more patent 
steps in the evolution of one species from another, and between 
them all there are so many shades of variation for which no words 
have yet been coined, that the naturalist who takes a comprehen- 
sive view of life upon our planet finds that what we have chosen 
to call species are often with difficulty separated from each other ; 
that, they have, in fact, no real existence in nature. All our 
classificatory divisions are more or less conventional. They are 
excellent as aids to thought and study, but misleading when be- 
