246 EN TOMOLOGY 
plished in the most forcible way, namely, by shattering the 
traditional belief in the immutability of species. Nowhere 
does Darwin imply that nature is striving to produce “ spe- 
cies’ for their own sake. <A process of evolution was the 
theme of Darwin and its key-note was adaptation. 
Indeed, for the purposes of the present generation, Dar- 
win’s immortal work would more properly be entitled—The 
Evolution of Adaptations by Means of Natural Selection. 
And to us, who now ridicule the old notion of the special 
creation of species, the doctrine of natural selection appears in 
a fresh light, with a new mission. For, in the words of 
Romanes, the theory is “ primarily, a theory of adaptations, 
and only becomes secondarily a theory of species in those com- 
paratively insignificant cases where the adaptations happen to 
be distinctive of the lowest order of taxonomic division.” 
The opposite view he compares “ to that of an astronomer who 
should define the nebular hypothesis as a theory of the origin 
of Saturn’s rings. It is indeed a theory of the origin of 
Saturn’s rings; but only because it is a theory of the origin of 
the entire solar system, of which Saturn’s rings form a part. 
Similarly, the theory of natural selection is a theory of the 
entire system of organic nature in respect of adaptations, 
whether these happen to be distinctive of particular species 
only, or are common to any number of species.” It should be 
remembered, of course, in using this comparison, that not all 
specific characters are adaptive. 
As regards the origin of species, however, there are several 
processes at work besides natural selection. Indeed, Darwin 
himself knew this, for he expressly stated: ‘‘ I am convinced 
that natural selection has been the most important, but not the 
exclusive, means of modification.” 
The Conception of ‘“Species.”—What is a “species”? 
The only practical criterion of species is isolation, or separate- 
ness, of one kind or another. The majority of our “ species ” 
are sharply separated from one another by structural differ- 
ences; the minority, however, blend into one another, and 
