As thus far developed, the nervous structure is one fitted only for a vague stimulation of dispersed contractile fibres, causing movements of an undirected kind. A concentration of these superficial nervous structures is a probable preliminary to the next change--an all-important change. For a part of the surface begins to sink inwards, forming, in the _Vertebrata_, a groove; and from the lining cells of this groove, which presently closes over, the central parts of the nervous system arise: definite nerves having meantime, as we may suppose, been developed out of the indefinite nervous plexus. Neglecting what there is in this of a speculative nature, it is sufficient for the present purpose to recognize the undoubted fact that the nervous system is developed from the ectoderm, and that, originally external, it is made internal by a process of sinking in or by a process of definite introversion. § 290. Whether direct equilibration or indirect equilibration has had the greater share in producing these fundamental contrasts between the inner and outer tissues of animals, must be left undecided. The two causes have all along co-operated--modification of the individual accumulated by inheritance predominating in some cases, and in other cases modification of the race by survival of the incidentally fittest. On the other hand, the action of the medium on the organism cannot fail to change its surface more than its centre, and so differentiate the two; while, on the other hand, the surfaces of organisms inhabiting the same medium display extreme unlikenesses which cannot be due to the immediate actions of their medium. Let us dwell a moment on the antithesis.