Living forms are, by their very definition, forms that are able to live. In whatever way the adaptation of the organism to its circumstances is explained, it has necessarily been sufficient, since the species has subsisted. In this sense, each of the successive species that paleontology and zoology describes was a _success_ carried off by life. But we get a very different impression when we refer each species to the movement that has left it behind on its way, instead of to the conditions into which it has been set. Often this movement has turned aside; very often, too, it has stopped short; what was to have been a thoroughfare has become a terminus. From this new point of view, failure seems the rule, success exceptional and always imperfect. We shall see that, of the four main directions along which animal life bent its course, two have led to blind alleys, and, in the other two, the effort has generally been out of proportion to the result.
Documents are lacking to reconstruct this history in detail, but we can make out its main lines. We have already said that animals and vegetables must have separated soon from their common stock, the vegetable falling asleep in immobility, the animal, on the contrary, becoming more and more awake and marching on to the conquest of a nervous system. Probably the effort of the animal kingdom resulted in creating organisms still very simple, but endowed with a certain freedom of action, and, above all, with a shape so undecided that it could lend itself to any future determination. These animals may have resembled some of our worms, but with this difference, however, that the worms living to-day, to which they could be compared, are but the empty and fixed examples of infinitely plastic forms, pregnant with an unlimited future, the common stock of the echinoderms, molluscs, arthropods, and vertebrates.
One danger lay in wait for them, one obstacle which might have stopped the soaring course of animal life. There is one peculiarity with which we cannot help being struck when glancing over the fauna of primitive times, namely, the imprisonment of the animal in a more or less solid sheath, which must have obstructed and often even paralyzed its movements. The molluscs of that time had a shell more universally than those of to-day. The arthropods in general were provided with a carapace; most of them were crustaceans. The more ancient fishes had a bony sheath of extreme hardness.[59] The explanation of this general fact should be sought, we believe, in a tendency of soft organisms to defend themselves against one another by making themselves, as far as possible, undevourable. Each species, in the act by which it comes into being, trends towards that which is most expedient. Just as among primitive organisms there were some that turned towards animal life by refusing to manufacture organic out of inorganic material and taking organic substances ready made from organisms that had turned toward the vegetative life, so, among the animal species themselves, many contrived to live at the expense of other animals. For an organism that is animal, that is to say mobile, can avail itself of its mobility to go in search of defenseless animals, and feed on them quite as well as on vegetables. So, the more species became mobile, the more they became voracious and dangerous to one another. Hence a sudden arrest of the entire animal world in its progress towards higher and higher mobility; for the hard and calcareous skin of the echinoderm, the shell of the mollusc, the carapace of the crustacean and the ganoid breast-plate of the ancient fishes probably all originated in a common effort of the animal species to protect themselves against hostile species. But this breast-plate, behind which the animal took shelter, constrained it in its movements and sometimes fixed it in one place. If the vegetable renounced consciousness in wrapping itself in a cellulose membrane, the animal that shut itself up in a citadel or in armor condemned itself to a partial slumber. In this torpor the echinoderms and even the molluscs live to-day. Probably arthropods and vertebrates were threatened with it too. They escaped, however, and to this fortunate circumstance is due the expansion of the highest forms of life.