And such a demand is made by Darwin. 'If,' says he, 'my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that, during these vast, yet quite unknown periods of time, the world teemed with living creatures.' This, however, we may say with the Regent Morton, is only 'a devout imagination'; and it might be more scientific to take the geological record as we find it, for, says Miller, 'it is difficult to imagine that that uniform cessation of organised life at one point, which seems to have conducted Sir Roderick Murchison and Professor Sedgwick to their conclusion, should thus have been a mere effect of accident. Accident has its laws, but uniformity is not one of them; and should the experience be invariable, as it already seems extensive, that immediately beneath the fucoidal beds organic remains cease, I do not see how the conclusion is to be avoided, that they represent the period in which, at least, _existences capable of preservation_ were first introduced.' Indeed the hypothesis of Darwin would fall under the remark of Herodotus, that the old theorisers and speculators at the last resort betook themselves to a belief in an imaginary ocean-river or to something in the interior of the earth where observation was of necessity excluded. For, as Professor Bain says, the assertion of a fact wholly beyond the reach of evidence for or against, is to be held as untrue: we are not obliged to show that a thing is not,--the burden lies on them who maintain that the thing is.