27. It must, however, be first stated, as a fact on which the assumption is founded which I have to notice, that the organic groups by which these successive strata are characterized, are not so distinct and separate, as it was convenient, for the sake of explanation, to describe them in the first instance. Although each body of strata is marked by predominant groups of genera and species, yet it is not true, that all the species of each formation disappear, when we proceed to the next. Some species and genera endure through several successive groups of strata; while others disappear, and new forms come into view, as we ascend. And thus, the change from one set of organic forms to another, as we advance in time, is made, not altogether by abrupt transitions, but in part continuously. The uniformitarian, in the case of organic, as in the case of mechanical change, obliterates or weakens the evidence of sudden and catastrophic leaps, by interposing intermediate steps, which involve, partly the phenomena of the preceding, and partly those of the subsequent condition. As he allows no universal transition from one deposit to a succeeding discrepant and unconformable deposit, so he allows no abrupt and complete transition from one collection of organic beings,--one creation, as we may call it,--to another. If creation must needs be an act out of the region of natural science, he will have it to be at least an act not exercised at distant intervals, and on peculiar occasions; but constantly going on, and producing its effects, as much at one time in the geological history of the world, as at another.