362 COMPARATIVE PHYSIOLOGY. 



parallel with the natural processes except in a very general 

 way. When we take into account the absence of muscular 

 movements, regulated according to no rigid principles, but vary- 

 ing with innumerable circumstances in all probability ; the ab- 

 sence of the influence of the nervous system determining the 

 variations in the quantity and composition of the outflow of the 

 secretions; the changes in the rate of so-called absorption, 

 which doubtless influences also the act of the secretion of the 

 juices — by these and a host of other considerations we are led 

 to hesitate before we commit ourselves too unreservedly to the 

 belief that the processes of natural digestion can be exactly 

 imitated in the laboratory. 



What is it which enables one animal to digest habitually 

 what may be almost a poison to another ? How is it that each 

 one can dispose readily of a food at one time that at another is 

 quite indigestible ? To reply that in the one case, the digestive 

 fluids are poured out and in the other not, is to go little below 

 the surface, for one asks the reason of this, if it be a fact, as it 

 no doubt is. When we look further into the peculiarities of 

 digestion, etc., we recognize the influence of race as such, and 

 in the race and the individual that obtrusive though ill-under- 

 stood fact — the force of habit — operative here as elsewhere. 

 And there can be little doubt that the habits of animals, as to 

 food eaten and digestive peculiarities established, become or- 

 ganized, fixed, and transmitted to posterity. 



It is probably in this way that, in the course of the evolu- 

 tion of the various groups of animals, they have come to vary 

 so much in their choice of diet and in their digestive processes, 

 did we but know them thoroughly as they are; for to assume 

 that even the digestion of mammals can be summed up in the 

 simple way now prevalent seems to us too broad an assump- 

 tion. The field is very wide, and as yet but little explored. 



The law of rhythm is illustrated, both in health and disease, 

 in striking ways in the digestive tract. An animal long accus- 

 tomed to eat at a certain hour of the day will experience at that 

 time not only hunger, but other sensations, probably referable 

 to secretion of a certain quantity of the digestive juices and to 

 the movements that usually accompany the presence of food in 

 the alimentary tract. Hence that '' colic " so common in horses 

 fed at irregular times and unwisely, after excessive work, etc. 



It is well known that defecation at periods fixed, even within 

 a few minutes, has become an established habit with hosts of 



