When we come to smaller divisions than the sub-kingdoms—­such as classes, orders, and families—­a similar succession 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 different orders of a class, in the main succeed one another in the relative order of their zoological rank—­the lower groups appearing 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 commencement 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; and though there are apparent exceptions to the rule, there are none of such a nature as not to admit of explanation.  Some of the leading facts upon which this generalisarion is founded will be enumerated immediately; but it will be well, in the first place, to consider briefly what we precisely mean when we speak of “higher” and “lower” groups.

It is well known that naturalists are in the habit of “classifying” the innumerable animals which now exist upon the globe; or, in other words, of systematically arranging them into groups.  The precise arrangement adopted by one naturalist may differ in minor details from that adopted by another; but all are agreed as to the fundamental points of classification, and all, therefore, agree in placing certain groups in a certain sequence.  What, then, is the principle upon which this sequence is based?  Why, for example, are the Sponges placed below the Corals; these below the Sea-urchins; and these, again, below the Shell-fish?  Without entering into a discussion of the principles of zoological classification, which would here be out of place, it must be sufficient to say that the sequence in question is based upon the relative type of organisation of the groups of animals classified.  The Corals are placed above the Sponges upon the ground that, regarded as a whole, the plan or type of structure of a Coral is more complex than that of a Sponge.  It is not in the slightest degree that the Sponge is in any respect less highly organised or less perfect, as a Sponge, than is the Coral as a Coral.  Each is equally perfect in its own way; but the structural pattern of the Coral is the highest, and therefore it occupies a higher place in the zoological scale.