The history of the mollusks, or animals inhabiting shells, furnishes us with an example still more striking. These animals, as they now exist, are divisible into the two great classes of carnivorous and herbivorous species, being distinguished by their anatomical structure; and so has it ever been. In the fossiliferous rocks below the tertiary, we find immense numbers of nautili, ammonites, and other kindred genera of polythalamous shells, called cephalopods, which were all carnivorous. And when they nearly disappeared with the cretaceous period, there was created another race with carnivorous propensities and organs, called trachelipods; and those continue still to swarm in the ocean. Had they not appeared when the cephalopods passed away, the herbivorous tribes would have multiplied to such an extent as ultimately to destroy marine vegetation, and bring on famine among themselves.
These examples are sufficient to prove the existence of the carnivorous and herbivorous races in all ages and in about the same relative numbers. And it certainly furnishes most decisive evidence of the oneness of all these systems of organic life on the globe.
_In the fifth place, the laws of anatomy have always been the same since organic structures began to exist._
It had long been known that the organs of animals were beautifully adapted to perform the functions for which they were intended. But it was not till the investigations of Baron Cuvier, within the last half century, that it was known how mathematically exact is the relation between the different parts of the animal frame, nor how precise are the laws of variation in the different species, by which they are fitted to different elements, climates, and food. It is now well known, that each animal structure contains a perfect system of correlation, and yet the whole forms a harmonious part of the entire animal system on the globe. But the language of Cuvier himself will best elucidate this subject, so far as it is capable of popular explanation.
"Every organized individual," says he, "forms an entire system of its own; all the parts of which mutually correspond, and concur to produce a certain definite purpose, by reciprocal reaction, or by combining towards the same end. Hence none of these separate parts can change their forms without a corresponding change in the other parts of the same animal, and consequently each of these parts, taken separately, indicates all the other parts to which it has belonged. Thus, if the viscera of any animal are so organized as only to be fitted for the digestion of recent flesh, it is also requisite that the jaws should be so constructed as to fit them for devouring prey; the claws must be constructed for seizing and tearing it to pieces; the teeth for cutting and dividing its flesh; the entire system of the limbs, or organs of motion, for pursuing and overtaking it; and the organs of sense, for discovering it at a distance. Nature, also, must have endowed the brain of the animal with instinct sufficient for concealing itself, and for laying plans to catch its necessary victims.