was brought about?  The long series of fossiliferous deposits, with their almost countless organic remains, is the link between what has been and what is; and if any answer to the above question can be arrived at, it will be by the careful and conscientious study of the facts of Palaeontology.  In the present state of our knowledge, it may be safely said that anything like a dogmatic or positive opinion as to the precise sequence of living forms upon the globe, and still more as to the manner in which this sequence may have been brought about, is incapable of scientific proof.  There are, however, certain general deductions from the known facts which may be regarded as certainly established.

In the first place, it is certain that there has been a succession of life upon the earth, different specific and generic types succeeding one another in successive periods.  It follows from this, that the animals and plants with which we are familiar as living, were not always upon the earth, but that they have been preceded by numerous races more or less differing from them.  What is true of the species of animals and plants, is true also of the higher zoological divisions; and it is, in the second place, quite certain that there has been a similar succession in the order of appearance of the primary groups ("sub-kingdoms,” “classes,” &c.) of animals and vegetables.  These great groups did not all come into existence at once, but they made their appearance successively.  It is true that we cannot be said to be certainly acquainted with the first absolute appearance of any great group of animals.  No one dare assert positively that the apparent first appearance of Fishes in the Upper Silurian is really their first introduction upon the earth:  indeed, there is a strong probability against any such supposition.  To whatever extent, however, future discoveries may push back the first advent of any or of all of the great groups of life, there is no likelihood that anything will be found out which will materially alter the relative succession of these groups as at present known to us.  It is not likely, for example, that the future has in store for us any discovery by which it would be shown that Fishes were in existence before Molluscs, or that Mammals made their appearance before Fishes.  The sub-kingdoms of Invertebrate animals were all represented in Cambrian times—­and it might therefore be inferred that these had all come simultaneously into existence; but it is clear that this inference, though incapable of actual disproof, is in the last degree improbable.  Anterior to the Cambrian is the great series of the Laurentian, which, owing to the metamorphism to which it has been subjected, has so far yielded but the singular Eozooen.  We may be certain, however, that others of the Invertebrate sub-kingdoms besides the Protozoa were in existence in the Laurentian period; and we may infer from known analogies that they appeared successively, and not simultaneously.