he exception _tests_ the rule. (Of course, the usual
translation "proves the rule," in the restricted modern sense of
"prove," is absurd.) That is to say, it appears on examination: (1) that
the alleged exception is not really one, and (2) that it stands in such
relation to the rule as to confirm it. For to all the above objections
it is replied that, granting the phenomenon in question (special
protective colouring for the female) to be absent, the alleged cause
(need of protection) is also absent; so that the proof is, by means of
the objections, extended, from being one by the method of Agreement,
into one by the Double Method.

Thirdly, an hypothesis originally intended to account for the whole of a
phenomenon and failing to do so, though it cannot be established in that
sense, may nevertheless contain an essential part of the explanation.
The Neptunian Hypothesis in Geology, was an attempt to explain the
formation of the Earth's outer crust, as having been deposited from an
universal ocean of mud. In the progress of the science other causes,
seismic, fluvial and atmospheric, have been found necessary in order to
complete the theory of the history of the Earth's crust; but it remains
true that the stratified rocks, and some that have lost their stratified
character, were originally deposited under water. Inadequacy, therefore,
is not a reason for entirely rejecting an hypothesis or treating it as
illegitimate.

(3) Granting that the hypothetical cause is real and adequate, the
investigation is not complete. Agreement with the facts is a very
persuasive circumstance, the more so the more extensive the agreement,
especially if no exceptions are known. Still, if this is all that can be
said in favour of an hypothesis, it amounts to proof at most by the
method of Agreement; it does not exclude the possibility of vicarious
causes; and if the hypothesis proposes a new agent that cannot be
directly observed, an equally plausible hypothesis about another
imagined agent may perhaps be invented.

According to Whewell, it is a strong mark of the truth of an hypothesis
when it agrees with distinct inductions concerning different classes of
facts, and he calls this the 'Consilience of Inductions,' because they
jump together in the unity of the hypothesis. It is particularly
convincing when this consilience takes place easily and naturally
without necessitating the mending and tinkering of the hypothesis; and
he cites the Theo