Very rapidly this conception of Natural Selection was extended by
naturalists until it came to be regarded as the general process of
life. They came to realise that all species, all genera, all classes of
life, whatever else may be happening to them, are and always have been
varying through the process of Natural Selection, some rapidly, some
slowly; some so slowly as hardly to change at all through vast ages. I
have stated the _a priori_ case by which, given birth and death and
individuality and changing conditions and sufficient time, it appears
logically inevitable that the change and differentiation of species
_must_ occur, and must be now going on. If we had no material evidence
at all it would still be possible to infer the evolution of species.
That _a priori_ case has never been answered, and it seems to me
unanswerable. But scientific men, with their obstinate preference for
observation and experiment over mere logical gymnastics, rarely rest
their convictions on _a priori_ cases. A sustaining scepticism is a
matter of conscience with them. To them an _a priori_ case is merely
a theory--that is to say, a generalisation under trial. For nearly
three-quarters of a century, therefore, biologists have been examining
whatever instances they could discover that seemed to contradict this
assumption that the process of specific change under Natural Selection
is the general condition of life. To this day this view is still called
the Theory of Natural Selection, though to a great number it has come
to have the substantial quality of an embracing fact.
It would have been amusing if Mr. Belloc had told us more of his ideas
of the scientific world. Apparently he knows scarcely anything of
museums or laboratories or the spirit and methods of research. And
manifestly he has not the faintest suspicion of the way in which the
whole world of vital phenomena has been ransacked and scrutinised to
test, correct, supplement, amplify, or alter this great generalisation
about life. He probably shares the delusion of most other men in the
street, that scientific theories are scientific finalities, that
they are supposed to be as ultimate as the dogmas of some infallible
religion. He imagines them put over chiefly by asseveration, just as
the assertions of a polemical journalist are put over. He has still to
learn that theories are trial material, testing targets, directives for
research. Shooting at established theories is the normal occupation
of the scientific investigator. Mr. Belloc’s figure of the scientific
investigator is probably a queer, frowsty, and often, alas! atheistical
individual, poking about almost aimlessly among facts in the hope of
hitting upon some “discovery” or “getting rid of a God.” He does not
understand the tense relevance of the vast amount of work in progress.
But for three-quarters of a century the thought and work of myriads
of people round and about the world have borne directly or almost
directly upon the probing, sounding, testing, of the theory of Natural
Selection. It stands clarified and, it would seem, impregnable to-day.