In all this, it may be said, the fact of evolution has been taken for
granted, but what are the evidences? Perhaps it should be frankly
answered that the idea of evolution, that the present is the child of
the past and the parent of the future, cannot be _proved_ as one may
prove the Law of Gravitation. All that can be done is to show that it is
a key--a way of looking at things--that fits the facts. There is no lock
that it does not open.
But if the facts that the evolution theory vividly interprets be called
the evidences of its validity, there is no lack of them. There is
_historical_ evidence; and what is more eloquent than the general fact
that fishes emerge before amphibians, and these before reptiles, and
these before birds, and so on? There are wonderfully complete fossil
series, e.g. among cuttlefishes, in which we can almost see evolution in
process. The pedigree of horse and elephant and crocodile is in general
very convincing, though it is to be confessed that there are other cases
in regard to which we have no light. Who can tell, for instance, how
Vertebrates arose or from what origin?
There is _embryological_ evidence, for the individual development often
reads like an abbreviated recapitulation of the presumed evolution of
the race. The mammal's visceral clefts are tell-tale evidence of remote
aquatic ancestors, breathing by gills. Something is known in regard to
the historical evolution of antlers in bygone ages; the Red Deer of
to-day recapitulates at least the general outlines of the history. The
individual development of an asymmetrical flat-fish, like a plaice or
sole, which rests and swims on one side, tells us plainly that its
ancestors were symmetrical fishes.
There is what might be called _physiological_ evidence, for many plants
and animals are variable before our eyes, and evolution is going on
around us to-day. This is familiarly seen among domesticated animals and
cultivated plants, but there is abundant flux in Wild Nature. It need
hardly be said that some organisms are very conservative, and that
change need not be expected when a position of stable equilibrium has
been secured.
There is also _anatomical_ evidence of a most convincing quality. In the
fore-limbs of backboned animals, say, the paddle of a turtle, the wing
of a bird, the flipper of a whale, the fore-leg of a horse, and the arm
of a man; the same essential bones and muscles are used to such diverse
results! What could it mean save blood relationship? And as to the two
sets of teeth in whalebone whales, which never even cut the gum, is
there any alternative but to regard them as relics of useful teeth which
ancestral forms possessed? In short, the evolution theory is justified
by the way in which it works.
§ 2