THE OUTER TISSUES OF ANIMALS. 307 



habitually 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 propa 

 gating 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 physi 

 cally 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 assimi 

 lating 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 de 

 velopment. The like holds with the linings of all internal 

 cavities and canals that have external openings. 



The transition from the literally outer tissues to those 

 tissues that are intermediate between them and the truly 

 inner tissues, is visible at all the orifices of the body ; where 

 skin and mucous membrane are continuous, and the one 

 passes insensibly into the other. This visible continuity is 

 not simply associated with a great degree of morphological 

 continuity, but also with a great degree of physiological con- 



