From what has just been said, it will be apparent that the question in debate is not merely a question of fact which can be settled by a direct appeal to observation. If this were the case, systematic naturalists could soon settle the question by their detailed knowledge of the structures which are severally distinctive of any given group of species. But so far is this from being the case, that systematic naturalists are really no better qualified to adjudicate upon the matter than are naturalists who have not devoted so much of their time to purely diagnostic work. The question is one of general principles, and as such cannot be settled by appeals to special cases. For example, suppose that the rest of this chapter were devoted to a mere enumeration of cases where it appears impossible to suggest the utility of certain specific characters, although such cases could be adduced by the thousand, how should I be met at the end of it all? Not by any one attempting to suggest the utility, past or present, of the characters named; but by being told that they must all present some _hidden_ use, must be _vestigial_, or else must be due to _correlation_. By appealing to one or other of these assumptions, our opponents are always able to escape the necessity of justifying their doctrine in the presence of otherwise inexplicable facts. No matter how many seemingly "indifferent characters" we may thus accumulate, Mr. Wallace and his followers will always throw upon us the impossible burden of proving the negative, that these apparently useless characters do _not_ present some hidden or former use, are _not_ due to correlation, and therefore have _not_ been produced by natural selection. It is in vain to retort that the burden of proof really lies the other way, or on the side of those who affirm that there is utility where no man can see it, or that there is correlation where no one can detect it. Thus, so far as any appeal to particular facts is concerned, it does not appear that there is any _modus vivendi_. Our opinions upon the question are really determined by the views which we severally take on matters of general principle. The issue, though it has a biological bearing, is a logical issue, not a biological one: it turns exclusively on those questions of definition and deduction with which we have just been dealing.