VII “THE ORIGIN OF SPECIES ” 229 
champions of its leading doctrines, or at any rate 
abstain from opposing them ; a host of young and 
ardent investigators seek for and find inspiration 
and guidance in Mr. Darwin’s great work ; and the 
general doctrine of evolution, to one side of which 
it gives expression, obtains, in the phenomena of 
biology, a firm base of operations whence it may 
conduct its conquest of the whole realm of Nature. 
History warns us, however, that it is the cus- 
tomary fate of new truths to begin as heresies and 
to end as superstitions ; and, as matters now stand, 
it is hardly rash to anticipate that, in another 
twenty years, the new generation, educated under 
the influences of the present day, will be in danger 
of accepting the main doctrines of the “ Origin of 
Species,” with as little reflection, and it may be 
_with as little justification, as so many of our con- 
temporaries, twenty years ago, rejected them. 
- Against any such a consummation let us all 
devoutly pray; for the scientific spirit is of more 
value than its products, and irrationally held 
truths may be more harmful than reasoned errors. 
Now the essence of the scientific spirit is criticism. 
It tells us that whenever a doctrine claims our 
assent we should reply, Take it if you can compel 
it. The struggle for existence holds as much in 
the intellectual as in the physical world.. A theory 
is a species of thinking, and its right to exist is 
coextensive with its power of resisting extinction 
by its rivals. 
