It is important to observe the limitations to this canon.  In the first place, it says that, unless adequate to explain the facts in question, an hypothesis cannot be ‘established’; but, for all that, such an hypothesis may be a very promising one, not to be hastily rejected, since it may take a very long time fully to verify an hypothesis.  Some facts may not be obtainable that are necessary to show the connection of others:  as, for example, the hypothesis that all species of animals have arisen from earlier ones by some process of gradual change, can be only imperfectly verified by collecting the fossil remains of extinct species, because immense depths and expanses of fossiliferous strata have been destroyed.  Or, again, the general state of culture may be such as to prevent men from tracing the consequences of an hypothesis; for which reason, apparently, the doctrine that the Sun is the centre of our planetary system remained a discredited hypothesis for 2000 years.  This should instruct us not to regard an hypothesis as necessarily erroneous or illegitimate merely because we cannot yet see how it works out:  but neither can we in such a case regard it as established, unless we take somebody’s word for it.

Secondly, 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