Their geological history has been very thoroughly studied; not only are we familiar with all their adult characters, but even their embryology is well known to naturalists.  It is, indeed, wonderful that the mode of growth of animals which died out in the Carboniferous period should be better known to us than that of many living types.  But it is nevertheless true that their embryonic forms have been found perfectly preserved in the rocks, and Barrande, in his “Systeme Silurien de la Boheme,” gives us all the stages of their development, from the time when the animal is merely sketched out as a simple furrow in the embryo to its mature condition.  So complete is the sequence, that the plate on which their embryonic changes are illustrated contains more than thirty figures, all representing different phases of their growth.  There is not a living Crab represented so fully in any of our scientific works as is that one species of Trilobite whose whole story Barrande has traced from the egg to its adult size.  Such facts should make those who rest their fanciful theories of the origin and development of life on the imperfection of the geological record, filling up the supposed lapses to suit themselves, more cautious as to their results.

We have found, then, Radiates, Mollusks, and Articulates in plenty; and now what is to be said of Vertebrates in these old times,—­of the highest and most important division of the Animal Kingdom, that to which we ourselves belong?  They were represented by Fishes alone; and the Fish chapter in the history of the early organic world is a curious, and, as it seems to me, a very significant one.  We shall find no perfect specimens; and he would be a daring, not to say a presumptuous thinker, who would venture to reconstruct a fish of the Silurian age from any remains that are left to us.  But still we find enough to indicate clearly the style of those old fishes, and to show, by comparison with the living types, to what group of modern times they belong.  We should naturally expect to find the Vertebrates introduced in their simplest form; but this is by no means the case:  the common fishes, as Cod, Herring, Mackerel, and the like, were unknown in those days.

But there are two groups of so-called fishes, differing from these by some marked features, among which we may find the modern representatives of these earliest Vertebrates.  Of these two groups one consists chiefly now of the Gar-Pikes of our Western waters, though the Sturgeons share also in some of their features.  In these fishes there is a singular union of reptilian with fish-like characters.  The systems of circulation and of respiration in them are more complicated than in the common fishes; the structure of the skull resembles that of the skull in reptiles, and they have other reptilian characters, such as their ability to move the head upon the neck independently of the body, and the connection of the vertebrae by ball-and-socket