To this question of theoretical interpretation, therefore, we must next address ourselves. And here, first of all, I should like to point out how sturdy must be the antecedent conviction of our opponents, if they are to maintain it in the face of such facts as have just been adduced. It must be remembered that this antecedent conviction is of a most uncompromising kind. By its own premisses it is committed to the doctrine that _all_ specific characters, without a single exception, _must_ be either useful, vestigial, or correlated. Well, if such be the case, is it not somewhat astonishing that out of 474 differences of colour which are distinctive of the 23 species of the genus Macropus, no single one appears capable of having any utility demonstrated, or indeed so much as suggested? For even the recent theory that slight differences of colour, which cannot be conceived as serving any other purpose, may enable the sexes of the same species quickly to recognize each other, is not here available. The species of the genus Macropus are more conspicuously distinguished by differences of size and form than by these minute differences of colour; and therefore no such use can be attributed to the latter. And, as previously stated, even within the order Marsupialia the genus Macropus is not at all exceptional in this respect; so that by including other genera of the order it would be easy to gather such apparently indifferent specific characters by the hundred, without any one of them presenting evidence--or even suggestion--of utility. How robust therefore is the faith of an _a priori_ conviction which can stand against such facts as these! What, then, are the _a priori_ grounds on which it stands? Mr. Wallace, the great leader of this school of thought, says:-- "It is a necessary deduction from the theory of natural selection, that none of the definite facts of organic nature, no special organ, no characteristic form or marking, no peculiarities of instinct or of habit, no relations between species or between groups of species, can exist, but which must now be, or once have been, _useful_ to the individuals or the races which possess them[89]." [89] _Contributions to the Theory of Natural Selection_, p. 47 (1870); republished in 1892.