Whether these differentiations are due wholly to indirect equilibration, or whether direct equilibration has had a share in working them, are questions that must be left open. Possibly a short hair so placed on a mammal’s face as to be very often touched, may, by conveying excitations to the nerves and vessels at its root, cause extra growth of the bulb and its appendages, and so the development of a _vibrissa_ may be furthered. Possibly, too, the light itself, to which the tissues of some inferior animals are everywhere sensitive, may aid in setting up certain of the modifications by which the nervous parts of visual organs are formed: producing, as it must, the most powerful effects at those points on the surface which the movements of the animal expose to the greatest and most frequent contrasts of light and shade; and propagating from those points currents of molecular change through the organism. But it seems clear that the complexities of the sensory organs are not thus explicable. They must have arisen by the natural selection of favourable variations. § 296. A group of facts, serving to elucidate those put together in the several foregoing sections, has to be added. I have reserved this group to the last, partly because it is transitional--links the differentiations of the literally outer tissues with those of the truly inner tissues. Though physically internal, the mucous coat of the alimentary canal has a _quasi_-externality from a physiological point of view. As was pointed out in the last chapter, the skin and the assimilating surface have this in common, that they come in direct contact with matters not belonging to the organism; and we saw that along with this community of relation to alien substances, there is a certain community of structure and development. The like holds with the linings of all internal cavities and canals that have external openings.