Analogous conclusions may be drawn respecting the processes of differentiation by which from the outer layer nervous tissue and finally a nervous system are evolved. Here, also, both direct and indirect equilibration appear to have operated. Two reasons may be assigned for the belief that the transformation of certain superficial cells into sensitive cells was initiated by exposure to external stimuli. The first is that, extremely unstable as protoplasm is, disturbances received by the outer side of a specially-exposed cell could scarcely fail to cause changes passing through it towards the interior mass of the body, and that perpetual repetition of such changes would tend to generate channels of easy transmission through the protoplasm. The second reason is that, if we do not assume this process of initiation but assume that survival of the fittest was the sole agency, then no reason can be assigned why the nervous system should not have been at the outset formed internally instead of being initiated externally and then transferred to the interior: the roundabout process would be inexplicable. At the same time the production of a central nervous system by introversion of superficial sensitive cells cannot be ascribed to the differentiating effects of external stimuli, but must be ascribed to natural selection. No perpetual repetition of outer disturbances would cause the sinking inwards, and covering up, of the specially-sensitive area and the plexus below it. But it is manifest that since these nervous structures, at once all-important and easily injured, would be safer if removed from the surface, survival of the fittest, continually preserving those in which they were more deeply seated, would tend to produce an arrangement in which all parts but the actual receivers of external stimuli became internal. Hence, contemplating generally these two fundamental differentiations of inner from outer tissues, we may conclude that though their first stages resulted from direct equilibration, their subsequent and higher stages resulted from indirect equilibration. CHAPTER VII. DIFFERENTIATIONS AMONG THE OUTER TISSUES OF ANIMALS. § 291. The outer tissues of animals, originally homogeneous over their whole surfaces, pass into a heterogeneity which fits their respective parts to their respective conditions. So numerous and varied are the implied differentiations, that it is impracticable here to deal with them all even in outline. To trace them up through classes of animals of increasing degrees of aggregation, would carry us into undue detail.