The problem is an undeniably difficult one, and it will not be possible here to give more than a mere outline of the modern views upon the subject.

Without entering into the at present inscrutable question as to the manner in which new life-forms are introduced upon the earth, it may be stated that almost all modern geologists hold that the living beings of any given formation are in the main modified forms of others which have preceded them.

It is not believed that any general or universal destruction of life took place at the termination of each geological period, or that a general introduction of new forms took place at the commencement of a new period.
It is, on the contrary, believed that the animals and plants of any given period are for the most part (or exclusively) the lineal but modified descendants of the animals and plants of the immediately preceding period, and that some of them, at any rate, are continued into the next succeeding 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.