THE INNER TISSUES OF ANIMALS. 315 



apparatus in its mouth, to gain by the habit of hurriedly 

 swallowing unmasticated food, it must also have the habit of 

 regurgitating the food for subsequent mastication. This 

 correlation of habits with their answering structures, may, as 

 we shall see, arise in a very simple way. The 



starting point of the explanation is a familiar fact — the fact 

 that indigestion, often resulting from excess of food, is apt to 

 cause that reversed peristaltic action known as vomiting. 

 From this we pass to the fact, also within the experience of 

 most persons, that during slight indigestion the stomach some- 

 times quietly regurgitates a small part of its contents as far 

 as the back of the mouth — giving an unpleasant acquaintance 

 with the taste of the gastric juices. Exceptional facts of the 

 same class help the argument a step further. " There are 

 certain individuals who are capable of returning, at will, a 

 greater or smaller portion of the contents of the digesting 

 stomach into the cavity of the mouth. * * * In some of these 

 cases, the expulsion of the food has required a violent effort. In 

 the majority, it has been easily evoked or suppressed. While 

 in others, it has been almost uncontrollable ; or its non- 

 occurrence at the habitual time has been followed by a 

 painful feeling of fulness, or by the act of vomiting." 

 Here then we have a certain physiological action, occa- 

 sionally happening in most persons and in some developed 

 into a habit more or less pronounced : indigestion being the 

 habitual antecedent. Suppose then that gregarious 



animals, living on innutritive food such as grass, are subject to 

 a like physiological action, and are capable of like varia- 

 tions in the degree of it. What will naturally happen ? 

 They wander in herds, now over places where food is scarce 

 and now coming to places where it is abundant. Some mas- 

 ticate their food completely before swallowing it ; while some 

 masticate it incompletely. If an oasis, presently bared by 

 their grazing, has not supplied the whole herd a full meal, 

 then the individuals which masticate completely will have 

 had less than those which masticate incompletely — will not 



