382 PHYSIOLOGICAL DEVELOPMENT. 



as take food into their interiors, are subject to forces of 

 another class tending to destroy their original homogeneity. 

 Food is a foreign substance which acts on the interior as an 

 environing object which touches it acts on the exterior — is 

 literally a portion of the environment, which, when swal- 

 lowed, becomes a cause of internal differentiations as the rest 

 of the environment continues a cause of external differentia- 

 tions. How essentially parallel are the two sets of actions 

 and reactions, we have seen implied by the primordial identity 

 of the endoderm and ectoderm in simple animals, and of the 

 skin and mucous membrane in complex animals (§§ 288, 289). 

 Here we have further to observe that as food is the original 

 source of internal differentiations, these may be expected to 

 show themselves first where the influence of the food is 

 greatest ; and to appear later in proportion as the parts are 

 more removed from the influence of the food. They do this. 

 In animals of low type, the coats of the alimentary cavity or 

 canal, are more differentiated than the tissue that lies between 

 the alimentary canal and the wall of the body. This tissue 

 in the higher Ccelenterata, is a feebly-organized parenchyma 

 traversed by lacunae — either simple channels, or canals lined 

 with simple ciliated cells ; and in the lower Mollusca the 

 structures bounding the perivisceral cavity and its ramifying 

 sinuses, are similarly imperfect. Further, it is observable 

 that the differentiation of this perivisceral sac and its sinuses 

 into a vascular system, proceeds centrifugally from the 

 region where the absorbed nutriment enters the mass of cir- 

 culating liquid, and where this liquid is qualitatively more 

 unlike the tissues than it is at the remoter parts of the body. 

 Physiological development, then, is initiated by that insta- 

 bility of the homogeneous which we have seen to be every- 

 where a cause of evolution (First Principles, §§ 109 — 115) . That 

 the passage from comparative uniformity of composition and 

 minute structure to comparative multiformity, is set up in 

 organic aggregates, as in all other aggregates, by the neces- 

 sary unlikenesses of the actions to which the parts are sub- 





