Man occupies the highest point in the vertebrate scale, for he breaks the chain of instincts and thus assures the complete expansion of his intellect. The insects, especially Hymenoptera, hold the same dominating position in the scale of the articulates, where they are the crowning point of instinctive life. These two groups represent the actual extremes of the two paths followed by psychic evolution in the Animal Kingdom,-the articulates toward instinct, the vertebrates toward intelligence. These two paths are quite divergent, but why have they diverged? At the beginning of their evolution, during that far distant epoch when they were differentiating along four main lines (Echinoderms, mollusks, articulates, and vertebrates), animals were threatened by a danger,-"an obstacle," says Bergson, "that doubtless almost checked the progress of animal life. There is a peculiarity which we cannot help being struck with when we glance at the fauna of early times. The mollusks then were more universally provided with shells than those of to-day. The arthropods in general were provided with a carapace. . . . The oldest fishes had a bony covering of extreme hardness." But, "the animal which is shut up in a citadel or in a coat of mail is condemned to an existence of half-sleep. It is in this torpor that the Echinoderms and even the mollusks are living to-day.' 9 The arthropods and the vertebrates escaped from it, and to this happy circumstance we owe the present development of the highest forms of life.
In two directions, indeed, do we see the impulse of active life regaining the upper hand. The fishes exchanged their ganoid armor for scales. Long before them the insects had appeared, having also rid themselves of the armor that protected their ancestors. In both groups the inefficiency of the protective envelop was compensated for by an agility that enabled them to escape their enemies and also to take the offensive and to choose the place and time of the encounter.
These remarks are profoundly true, but they should be modified in one point which is essential to explain the structure and the special psychology of the articulates. These animals have never lost their chitinous armor that protected their primitive ancestors. They have preserved it in its entirety and with greater or less thickness, and the Coleoptera, the crabs, the scorpions, and the thousand-legs of our times yield nothing in this regard to the ancient forms from which they are descended.
In fact, to-day as formerly, they are covered with an external skeleton of chitin, and that is why Edmund Perrier, wishing to emphasize their dominant character, has called them by the name of Chitinophores. To escape imprisonment within their protective envelop, to acquire the flexibility and mobility necessary to their evolution, they became the object of certain superficial modifications. These consisted in the division of the armor into several pieces by means of articular fines, along which the chitin is less thick than elsewhere, dividing their armor into pieces and permitting these pieces to move one upon another. Thus they became articulated, and at once acquired agility without losing their protective armor. Naturally, such joints were formed wherever the several segments, arranged in a row and constituting the body of the animal, came together, with the result of giving these segments a certain independence and yet preserving, to a large degree, their uniformity. Indeed,, we see that many articulates possess a pair of appendages on each segment (myriapods and the majority of crustaceans) and that the insects most remote in this regard from the primitive types are still provided with seven pairs of appendages (one pair of antennae, three pairs of mouth parts, and three pairs of legs), without counting the modified or rudimentary organs to be seen at different points on the abdomen. And the chitinous envelop of these appendages has broken into joints just as the body is divided into rings. Hence the name of arthropods, often, given to articulate animals.
What a difference from the vertebrates, in which the skeleton becomes an internal framework which allows the organism to reach greater dimensions, the segments to fuse to a greater degree and to lose more or less their independence, all of which results in the reduction of the number of limbs to only two pairs Î
Now, the relative independence of the segments and the multiplicity of the appendages have as a corollary a differentiation of these parts, each of which plays a special role in the organism. As Bergson observes, the various appendages of the articulates are a kind of natural tools, which differ from one another in structure as well as in function. Their specialization may even be carried so far that each part of an organ performs a special function. This is well seen in the bee, in which the first tarsal joint of the hind legs is transformed into a brush, the tibia into a pollen basket, while the two joints, by the contact of their edges, form pincers which seize the flake of wax secreted by the abdomen underneath. It is an admirable tool and wonderfully adapted to fill its particular functions. As a general rule, apart from the changes which they undergo in the course of specific evolution, the appendages of arthropods are nearly unchangeable in the individual and are narrowly adapted to certain purposes ; they are the tools for instinctive work, in which they differ from the less specialized but more supple limbs that serve as implements to the vertebrates, at least to the higher vertebrates. With these latter, as Bergson says, the two pairs of limbs "perform functions much less strictly dependent upon their form," acquiring complete independence in man, "whose hand can do any kind of work." Thus, the extraordinary preponderance of instinctive activity among the articulates seems to have as its essential point of dependence the differentiation and the multiplicity of the appendages,-that is to say, the chitin-ization of the integument and the formation of articular lines which result from it. These animals from the beginning were doomed to use organic tools, and they made the best use of them. Their main psychical task was to grave upon their memory and to repeat instinctively the acts to which these organs were fitted.