THE INNER TISSUES OF ANIMALS. 329 



have had enough. Those which masticate incompletely and 

 distend their stomachs with food difficult to digest, will be 

 liable to these regurgitations ; but if they re-masticate what 

 is thus returned to the mouth (and we know that animals 

 often eat again what they have vomited), then the extra 

 quantity of food taken, eventually made digestible, will yield 

 them mor nourishment than is obtained by those which 

 masticate completely at first. The habit initiated in this 

 natural way, and aiding survival when food is scarce, will 

 be apt to cause modifications of the alimentary canal. 

 We know that dilatations of canals readily arise under 

 habitual distensions. We know that canals habitually 

 distended become gradually more tolerant of the contained 

 masses that at first irritated them. And we know that 

 there commonly take place adaptive modifications of their 

 surfaces. Hence if a habit of this kind and the structural 

 changes resulting from it, are in any degree inheritable, it is 

 clear that, increasing in successive generations, both imme- 

 diately by the cumulative effect of repetitions and mediately 

 by survival of the individuals in which they are most decided, 

 they may go on until they end in the peculiarities which 

 Ruminants display. 



298. There are structures belonging to the same group 

 which cannot, however, be accounted for in this way. They 

 are the organs that secrete special products facilitating diges- 

 tion the liver, pancreas, and various smaller glands. All 

 these appendages of the alimentary canal, large and inde- 

 pendent as some of them seem, really arise by differentia- 

 tions from its coats. The primordial liver consists of nothing 

 more than bile-cells scattered along a tract of the intestinal 

 surface. Accumulation of these bile-cells is accompanied by 

 increased growth of the surface which bears them a growth 

 which at first takes the form of a cul-de-sac, having an outside 

 that projects from the intestine into the peri-visceral cavity. 

 As the mass of bile-cells becomes greater, there arise se- 



