When we say that rain comes from the condensation
of moisture in the atmosphere, they demand of us a rain-drop from
moisture not yet condensed. If they stickle for proof and cavil on
the ninth part of a hair, as they do when we bring forward what we
deem excellent instances of the transmission of an acquired
characteristic, why may not we, too, demand at any rate some
evidence that the unmodified beetles actually did always, or nearly
always, get blown out to sea, during the reduction above referred
to, and that it is to this fact, and not to the masterly inactivity
of their fathers and mothers, that the Madeira beetles owe their
winglessness? If we begin stickling for proof in this way, our
opponents would not be long in letting us know that absolute proof
is unattainable on any subject, that reasonable presumption is our
highest certainty, and that crying out for too much evidence is as
bad as accepting too little. Truth is like a photographic
sensitized plate, which is equally ruined by over and by under
exposure, and the just exposure for which can never be absolutely
determined.
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