SUMMARY OF PHYSIOLOGICAL DEVELOPMENT. 389 



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 which lies be- 

 tween the alimentary canal and the wall of the body. This tis- 

 suein the higher Ccelenterata, is a feebly-organized parenchyma 

 traversed by 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 centri- 

 fugally from the region where the absorbed nutriment enters 

 the mass of circulating liquid, and where this liquid is quali- 

 tatively more unlike the tissues than it is at the remoter 

 parts of the body. 



Physiological development, then, is initiated by that in- 

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

 where a cause of evolution (First Principles, 149 155). 

 That the passage from comparative uniformity of composi- 

 tion and minute structure to comparative multiformity, is set 

 up in organic aggregates, as in all other aggregates, by the 

 necessary unlikenesses of the actions to which the parts are 

 subject, is shown by the universal rise of the primary differen- 

 tiation into the parts that are universally most contrasted in 

 their circumstances, and by the rise of secondary differen- 



