THE INNER TISSUES OF ANIMALS. 311 



states — implies that the surface with which it now comes in 

 contact is differently affected by it from the preceding sur- 

 faces — implies, that is, a differentiating action. To use con- 

 crete language ; — food that is broken down in the mouth acts 

 on the oesophagus and stomach in a way unlike that which 

 it would have done had it been swallowed whole ; the masti- 

 cated food, to which certain solvents or ferments are added, 

 becomes to the intestine a different substance from that which 

 it must have otherwise been ; and the altered food, resolved 

 by these additions into its proximate principles, cannot have 

 those proximate principles absorbed in the next part of the 

 intestine, without the remoter parts being affected as they 

 would not have been in the absence of absorption. It is true 

 that in developed alimentary canals, such as the reasoning 

 here tacitly assumes, these marked successive differentiations 

 of the food are themselves the results of pre-established 

 differentiations in the successive parts of the canal. But it is 

 also true that actions and reactions like those here so definitely 

 marked, must go on indefinitely in an undeveloped alimentary 

 canal. If the food is changed at all in the course of its transit, 

 which it must be if the creature is to live by it, then it 

 cannot but act dissimilarly on the successive tracts of the 

 alimentary canal, and cannot but be dissimilarly reacted on 

 by them. Inevitably, therefore, the uniformity of the surface 

 must lapse into greater or less multiformity : the differentia- 

 tion of each part tending ever to initiate differentiations of 

 other parts. 



Not, indeed, that the implied process of direct equilibra- 

 tion can be regarded as the sole process. Indirect equilibra- 

 tion aids ; and, doubtless, there are some of the modifications 

 which only indirect equilibration can accomplish. But we 

 have here one unquestionable cause — a cause that is known 

 to work in individuals, changes of the kind alleged. Where, 

 for instance, cancerous disease of the oesophagus so narrows 

 the passage into the stomach as to prevent easy descent of 

 the food, the oesophagus above the obstruction becomes 

 Vol. II. 14 



