These eye-facets form the sense of light, rather than organs of seeing. Their almost paradoxical number at least, and the singularity of their forms, render it probable that they impel the animal by some modification of its irritability, herein likewise containing a striking analogy to the known influence of light on plants, than as excitements of sensibility. The sense that is nearest akin to irritability, and which alone resides in the muscular system, is that of touch, or feeling. This, therefore, is the first sense that emerges. Being confined to absolute contact, it occupies the lowest rank; but for that very reason it is the ground of all the other senses, which act, according to the ratio of their ascent, at still increasing distances, and become more and more ideal, from the tentacles of the polypus, to the human eye; which latter might be defined the outward organ of the identity, or at least of the indifference, of the real and ideal. But as the calcareous residuum of the lowest class approaches to the nature of horn in the snail, so the cumbrous shell of the snail has been transformed into polished and moveable plates of defensive armour in the insect. Thus, too, the same power of progressive individuation articulates the tentacula of the polypus and holothuria into antennæ; thereby manifesting the full emersion and eminency of irritability as a power which acts in, and gives its own character to, that of reproduction. The least observant must have noticed the lightning-like rapidity with which the insect tribes devour and eliminate their food, as by an instinctive necessity, and in the least degree for the purposes of the animal's own growth or enlargement. The same predominance of irritability, and at the same time a new start in individuation, is shown in the reproductive power as generation. There is now a regular projection, _ab intra ad extra_, for which neither sprouts nor cuttings can any longer be the substitutes. We have not space for further detail; but there is one point too strikingly illustrative and even confirmative of the proposed system, to be omitted altogether. We mean the curious fact, that the same characteristic tendency, _ad extra_, which in the males and females of certain insect tribes is realized in the functions of generation, conception, and parturiency, manifests and expands itself in the _sexless_ individuals (which are always in this case the great majority of the species), as instincts of art, and in the construction of works completely detached and inorganic; while the geometric regularity of these works, which bears an analogy to crystallization, is demonstrably no more than the necessary result of uniform action in a compressed multitude.