368 HISTORICAL PALiEONTOLOGY. 



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 in- 

 ferred 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 Eozodii. 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 kno'\\Ti analogies that they appeared suc- 

 cessively, and not simultaneously. 



When we come to smaller divisions than the sub -king- 

 doms — such as classes, orders, and families — a similar suc- 

 cession of groups is observable. The different classes of 

 any given sub-kingdom, or the different orders of any given 

 class, do not make their appearance together and all at once, 

 but they are introduced upon the earth in succession. More 

 than this, the different classes of a sub-kingdom, or the differ- 

 ent orders of a class, i?i the maiti succeed 07ie another in the 

 relative order of their zoological rank — the lower groups appear- 

 ing first and the higher groups last. It is true that in the 

 Cambrian formation — the earliest series of sediments in which 

 fossils are abundant — we find numerous groups, some very 

 low, others very high, in the zoological scale, which appear 

 to have simultaneously flashed into existence. For reasons 

 stated above, however, we cannot accept this appearance as 

 real ; and we must believe that many of the Cambrian groups 

 of animals really came into being long before the commence- 

 ment of the Cambrian period. At any rate, in the long series 

 of fossiliferous deposits of later date than the Cambrian the 

 above-stated rule holds good as a broad generalisation — that 

 the lower groups, namely, precede the higher in point of time ; 



