Of the earliest known vertebrates,--the placoidal fishes of the Upper Silurian rocks,--we possess only fragments, which, however, sufficiently indicate, from their resemblance to the corresponding parts of an existing shark,--the cestracion,--that they belonged to fishes furnished with the two pairs of fins now so generally recognized as the homologues of the fore and hinder limbs in quadrupeds. With the second earliest vertebrates,--the ganoids of the Old Red Sandstone,--we are more directly acquainted, and know that they exhibited the true typical form,--a vertebral column terminating in a brain-protecting skull; and that, in at least the acanth, celacanth, and dipterian families, they had the limb-like fins. In the upper parts of the system the earliest reptiles leave the first known traces of the typical foot, with its five digits. Higher still in one of the deposits of the Trias we are startled by what seems to be the impression of a human hand of an uncouth massive shape, but with the thumb apparently set in opposition, as in man, to the other fingers; we next trace the type upwards among the wonderfully developed reptiles of the Secondary periods; then among the mammals of the Tertiary ages, higher and yet higher forms appear; the mute prophecies of the coming being become with each approach clearer, fuller, more expressive, and at length receive their fulfilment in the advent of man. A double meaning attaches to the term type; and hence some ambiguity in the writings which have appeared on this curious subject. Type means a prophecy embodied in symbol; it means also what Sir Joshua Reynolds well terms "one of the general forms of nature,"--a pattern form, from which all others in the same class or family, however numerous, are recognized as mere exceptions and aberrations. But in the geologic series both meanings converge and become one. The form or number typical as the _general_ form or number, is found typical also as a _prophecy_ of the form or number that came at length to be exemplified in the deputed lord of creation. Let us in our examples take typical numbers, as more easily illustrated without diagrams than typical forms.