But though the nature of the agencies above considered has never changed, the intensity or amount of their action has varied; how much is a point not yet settled among geologists. Some regard that intensity, as it has existed during the present or alluvial period, as a standard for all preceding periods; that is, the intensity of these forces has never varied more during any period of the earth's history than it has since the alluvial period commenced. Most geologists, however, regard this as an extreme opinion, and think they see evidence in geology of a far greater intensity in these agencies in past periods than exists at present. They think they have proof that the world was once only a molten mass of matter, and some evidence that previously it was in a state of vapor. They believe that vast mountains, and even continents, have sometimes been thrown up from the ocean's bed by a single mighty paroxysmal effort; and such effects they know to be far greater than the causes of change now in operation can produce, without a vast increase of their intensity. But this question need neither be discussed nor decided for the sake of my present argument, since my object is to prove an identity in the nature and laws, not in the intensity, of geological agencies. _In the fourth place, the laws of zoology and botany have always been the same on the globe._ An examination of the animals now living, amounting to some hundred thousand species, perhaps to one or two millions, shows that they may be arranged in four great classes. The first class embraces the vertebral animals, distinguished by having a vertebral column, or back-bone, a regular skeleton, and a regular nervous system. It comprehends all the quadrupeds and bipeds, with man at their head, and is much superior to all other classes in complexity of organization and strength of the mental powers. The second class embraces the mollusks, or animals inhabiting shells. They are destitute of a spinal marrow, and for the most part their muscles are attached to the external covering, called the shell, although this shell is sometimes internal. The third class are called articulated animals, having envelopes connected by annulated plates, or rings. It includes such animals as the lobster, bloodsucker, spider, and insects generally. The fourth class have a radiated structure, and often resemble plants, or their habitation is a stony structure. Hence they are sometimes called zoophytes, which means _animal plants_; or lithophytes, which means _stony plants_. They swarm in the ocean, and some of them build up those extensive stony structures called coral reefs.