y, the canon says that an hypothesis is not established, unless
it accounts for the phenomena _so far as it professes to_. But it
implies a complete misunderstanding to assail a doctrine for not
explaining what lies beyond its scope. Thus, it is no objection to a
theory of the origin of species, that it does not explain the origin of
life: it does not profess to. For the same reason, it is no objection to
the theory of Natural Selection, that it does not account for the
variations which selection presupposes. But such objections might be
perfectly fair against a general doctrine of Evolution.

An interesting case in Wallace's _Darwinism_ (chap. x.) will illustrate
the importance of attending to the exact conditions of an hypothesis. He
says that in those groups of "birds that need protection from enemies,"
"when the male is brightly coloured and the female sits exposed on the
nest, she is always less brilliant and generally of quite sober and
protective hues"; and his hypothesis is, that these sober hues have been
acquired or preserved by Natural Selection, because it is important to
the family that the sitting bird should be inconspicuous. Now to this it
might be objected that in some birds both sexes are brilliant or
conspicuous; but the answer is that the female of such species _does not
sit exposed on the nest_; for the nests are either domed over, or made
in a hole; so that the sitting bird does not need protective colouring.
If it be objected, again, that some sober-coloured birds build domed
nests, it may be replied that the proposition 'All conspicuously
coloured birds are concealed in the nest,' is not to be converted simply
into 'All birds that sit concealed in the nest 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_--t