286 PHYSIOLOGICAL DEVELOPMENT. 



tration is here used merely to convey, in a way easy to 

 follow, an idea of the relations between outer and inner 

 tissues, as they exist in the more complex animals. The two 

 facts which we have to note are these : — First that, as Prof. 

 Huxley points out in his essay on " Tegumentary Organs," 

 the course of differentiation in the body- wall of the Hydra, i« 

 paralleled by the course of differentiation in the skin of every 

 more complex animal up to the highest mammal. Between 

 the epidermis and the derma there is a layer of indifferent 

 tissue corresponding to the layer that lies between the endo- 

 derm and ectoderm of the Hydra ; and from this layer, as 

 from its homologue, the differentiations proceed in opposite 

 directions. Though the resulting two layers, exposed to 

 more unlike conditions than those of the Hydra, are more 

 unlike one another, yet we see in them essentially the same 

 course of metamorphosis and the same subordination of it to 

 the relations of outside and inside. In the second place, we 

 have to note that the wall of the alimentary canal, though it 

 is in one sense internal by contrast with the skin as external, 

 and is correspondingly differentiated from the skin, is in 

 another sense like the skin, in having one surface in contact 

 with foreign substances (presented as food) and the other 

 surface in contact with the living substance of the body ; and 

 that consequently it undergoes, like the skin, a differentia- 

 tion into two layers, one growing towards the relatively 

 external or food-containing cavity, and the other towards the 

 rigorously internal cavity — the closed peri- visceral sac. 



§ 290. Whether direct equilibration or indirect equilibra- 

 tion has had the greater share in producing this universally- 

 present contrast between the inner and outer tissues of 

 animals, must be left undecided. The two causes have all 

 along co-operated — modification of the individual accumu- 

 lated by inheritance predominating in some cases, and in 

 other cases modification of the race by survival of the inci- 

 dentally fittest. On the one hand, the action of the medium 



