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If these things are so, the question naturally suggests itself: Why should certain types of animals have attained their greatest size at certain different epochs, and been replaced at others by equally big animals of wholly unlike sorts? The answer, I believe, is simply this: Because there is not room and food in the world at any one time for more than a certain relatively small number of gigantic species. Each great group of animals has had successively its rise, its zenith, its decadence, and its dotage; each at the period of its highest development has produced a considerable number of colossal forms; each has been supplanted in due time by higher groups of totally different structure, which have killed off their predecessors, not indeed by actual stress of battle, but by irresistible competition for food and prey. The great saurians were thus succeeded by the great mammals, just as the great mammals are themselves in turn being ousted, from the land at least, by the human species.

Let us look briefly at the succession of big animals in the world, so far as we can follow it from the mutilated and fragmentary record of the geological remains.

The very earliest existing fossils would lead us to believe what is otherwise quite probable, that life on our planet began with very small forms–that it passed at first through a baby stage. The animals of the Cambrian period are almost all small mollusks, star-fishes, sponges, and other simple, primitive types of life. There were as yet no vertebrates of any sort, not even fishes, far less amphibians, reptiles, birds, or mammals. The veritable giants of the Cambrian world were the crustaceans, and especially the trilobites, which, nevertheless, hardly exceeded in size a good big modern lobster. The biggest trilobite is some two feet long; and though we cannot by any means say that this was really the largest form of animal life then existing, owing to the extremely broken nature of the geological record, we have at least no evidence that anything bigger as yet moved upon the face of the waters. The trilobites, which were a sort of triple-tailed crabs (to speak very popularly), began in the Cambrian Epoch, attained their culminating point in the Silurian, waned in the Devonian, and died out utterly in the Carboniferous seas.

It is in the second great epoch, the Silurian, that the cuttle-fish tribe, still fairly represented by the nautilus, the argonaut, the squid, and the octopus, first began to make their appearance upon this or any other stage. The cuttle-fishes are among the most developed of invertebrate animals; they are rapid swimmers; they have large and powerful eyes; and they can easily enfold their prey (teste Victor Hugo) in their long and slimy sucker-clad arms. With these natural advantages to back them up, it is not surprising that the cuttle family rapidly made their mark in the world. They were by far the most advanced thinkers and actors of their own age, and they rose almost at once to be the dominant creatures of the primaeval ocean in which they swam. There were as yet no saurians or whales to dispute the dominion with these rapacious cephalopods, and so the cuttle family had things for the time all their own way. Before the end of the Silurian Epoch, according to that accurate census-taker, M. Barrande, they had blossomed forth into no less than 1,622 distinct species. For a single family to develop so enormous a variety of separate forms, all presumably derived from a single common ancestor, argues, of course, an immense success in life; and it also argues a vast lapse of time during which the different species were gradually demarcated from one another.

Some of the ammonites, which belonged to this cuttle-fish group, soon attained a very considerable size; but a shell known as the orthoceras (I wish my subject didn’t compel me to use such very long words, but I am not personally answerable, thank heaven, for the vagaries of modern scientific nomenclature) grew to a bigger size than that of any other fossil mollusk, sometimes measuring as much as six feet in total length. At what date the gigantic cuttles of the present day first began to make their appearance it would be hard to say, for their shell-less bodies are so soft that they could leave hardly anything behind in a fossil state; but the largest known cuttle, measured by Mr. Gabriel, of Newfoundland, was eighty feet in length, including the long arms.