are conspicuously coloured.’  In the cases alleged the domed nests are a protection against the weather, and the sober colouring is a general protection to the bird, which inhabits an open country.  It may be urged, however, that jays, crows, and magpies are conspicuous birds, and yet build open nests:  but these are aggressive birds, not needing protection from enemies.  Finally, there are cases, it must be confessed, in which the female is more brilliant than the male, and which yet have open nests.  Yes:  but then the male sits upon the eggs, and the female is stronger and more pugnacious!

Thus every objection is shown to imply some inattention to the conditions of the hypothesis; and in each case it may be said, exceptio probat regulam—­the 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.