816 PHYSIOLOGICAL DEVELOPMENT. 



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 more 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 habitu- 

 ally 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 

 digestion — the liver, pancreas, and various smaller glands. 

 All these appendages of the alimentary canal, large and 

 independent as some of them seem, really arise by differen- 

 tiations from its coats. The primordial liver, as we see it in 

 a simple animal such as the Planar ia, 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, 



