THE OUTER TISSUES OF ANIMALS. 321 



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-extem&lity 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 

 development. 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 which 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 

 associated not simply with a great degree of morphological 

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

 tinuity. That is to say, these literally outer and quasi-outer 

 layers are capable of rapidly assuming one another's struc- 

 tures and functions when subject to one another's conditions. 

 Mucous surfaces, normally kept covered, become skin-like if 

 exposed to the air; but resume more or less fully their 

 normal characters when restored to their normal positions. 

 These are truths familiar to pathologists. They continually 

 meet with proofs that permanent eversion of the mucous 

 membrane, even where it is by prolapse of a part deeply 

 seated within the body, is followed by an adaptation eventu- 

 ally almost complete: originally moist, tender to the touch, 



and irritated by the air, the surface gradually becomes covered 

 67 



