One other fact, showing the identity of former zoological laws with those which now prevail, must not be omitted. I refer to the existence on the globe in all past periods of organic life of the two great classes of carnivorous and herbivorous animals; and they have always existed, too, in about the same proportion. To the harmony and happiness of the present system, we know that the existence and proper relative number of these different classes are indispensable. For in order that the greatest possible number of animals that live on vegetable food should exist, they must possess the power of rapid multiplication, so that there should be born a much larger number than is necessary to people the earth. But if there existed no carnivorous races to keep in check this redundancy of population, the world would soon become so filled with the herbivorous races that famine would be the consequence, and thus a much greater amount of suffering result than the sudden death inflicted by carnivorous races now produces. To preserve, then, a proper balance between the different species is, doubtless, the object of the creation of the carnivorous. This system has been aptly denominated "the police of nature." And we find it to have always existed. The earliest vertebral animals--the sauroid fishes and sharks--were of this description. The sharks have always lived, but the sauroid fishes became less numerous when other marine saurians were created; and when they both nearly disappeared, during the tertiary period, other predaceous families were introduced, more like those now in existence. 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._