period, either unchanged, or so far altered as to appear as new species.  To discuss these views in detail would lead us altogether too far, but there is one very obvious consideration which may advantageously receive some attention.  It is obvious, namely, that the great discordance which is found to subsist between the animal life of any given formation and that of the next succeeding formation, and which no one denies, would be a fatal blow to the views just alluded to, unless admitting of some satisfactory explanation.  Nor is this discordance one purely of life-forms, for there is often a physical break in the successions of strata as well.  Let us therefore briefly consider how far these interruptions and breaks in the geological and palaeontological record can be accounted for, and still allow us to believe in some theory of continuity as opposed to the doctrine of intermittent and occasional action.

In the first place, it is perfectly clear that if we admit the conception above mentioned of a continuity of life from the Laurentian period to the present day, we could never prove our view to be correct, unless we could produce in evidence fossil examples of all the kinds of animals and plants that have lived and died during that period.  In order to do this, we should require, to begin with, to have access to an absolutely unbroken and perfect succession of all the deposits which have ever been laid down since the beginning.  If, however, we ask the physical geologist if he is in possession of any such uninterrupted series, he will at once answer in the negative.  So far from the geological series being a perfect one, it is interrupted by numerous gaps of unknown length, many of which we can never expect to fill up.  Nor are the proofs of this far to seek.  Apart from the facts that we have hitherto examined only a limited portion of the dry land, that nearly two-thirds of the entire area of the globe is inaccessible to geological investigation in consequence of its being covered by the sea, that many deposits can be shown to have been more or less completely destroyed subsequent to their deposition, and that there may be many areas in which living beings exist where no rock is in process of formation, we have the broad fact that rock-deposition only goes on to any extent in water, and that the earth must have always consisted partly of dry land and partly of water—­at any rate, so far as any period of which we have geological knowledge is concerned.  There must, therefore, always have existed, at some part or another of the earth’s surface, areas where no deposition of rock was going on, and the proof of this is to be found in the well-known phenomenon of “unconformability.”  Whenever, namely, deposition of sediment is continuously going on within the limits of a single ocean, the beds which are laid down succeed one another in uninterrupted and regular sequence.  Such beds are said to be “conformable,” and there are many rock-groups