There are vertebrate animals of the second age of ichthyic existence, that, like the _Pterichthys_ and _Coccosteus_, were furnished with but two limbs. The murænidæ of recent times have no more; at least one of their number, the muræna proper, wants limbs altogether; so also do the lampreys. The snakes are equally limbless, save that the boas and pythons possess the rudiments of a single pair; and such also is the condition, among the amphibia, of all the known species of Coecilia. And yet, notwithstanding these exceptional cases, the true typical number of limbs, as shown by a preponderating majority of the vertebrates of all ages of the world, is four. And this typical number is the human number. There is as certainly a typical number of digits too, as of the limbs which bear them. The exceptions are many. All the species of the horse genus possess but a single digit; the cattle family possess but two digits, the rhinoceros three digits, the hippopotamus four digits; many animals, such as the dog and cat, have but four digits on one pair of limbs and five on the other; whereas in some of the fishes the number of digits is singularly great,--from ten to twenty in most species, and in the rays from eighty to a hundred. And yet, as shown in the rocks, in which, however, the aberrations appear early, the true typical number is five on both the fore and hinder limbs. And such is the number in man. There is also, in at least the mammalia, a typical number of vertebræ in the neck. The three-toed sloth has nine cervical vertebræ; the manati only six; but seven is the typical number. And seven is the human number also. Man, in short, is pre-eminently what a theologian would term the antetypical existence,--the being in whom the types meet and are fulfilled. And not only do typical forms and numbers of the exemplified character meet in man, but there are not a few parts of his framework which in the inferior animals exist as but mere symbols, of as little importance as dugs in the male animal, though they acquire significancy and use in him. Such, for instance, are the many-jointed but moveless and unnecessary bones of which the stiff inflexible _fin_ of the dugong and the fore paw of the mole consist, and which exist in his arm as essential portions, none of which could be wanted, of an exquisitely flexible instrument. In other cases, the old types are exemplified serially in the growth and development of certain portions of his frame. Such is specially the case with that all important portion of it, the organ of thought and feeling. The human brain is built up by a wonderful process, during which it assumes in succession the form of the brain of a fish, of a reptile, of a bird, of a mammiferous quadruped; and, finally, it takes upon it its unique character as a human brain. Hence the remark of Oken, that "man is the sum total of all the animals;" hence, too, a recognition of type in the _history_ of the successive vertebral periods of the geologist,