368 HISTORICAL PALZONTOLOGY. 
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 ve/ative 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 ¢iese 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 Hozoou. 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 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, zz the main succeed one another in the 
relative order of their zoological rank 
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 ; 
