450 COMPARATIVE PHYSIOLOGY. 



SFECIAIi CONSIDERATIONS. 



If the student will now read afresh what has been written 

 under the above heading in relation to the subject of digestion, 

 it will probably appear in a new light. We endeavored to show 

 that, according to that general principle of correlation which 

 holds throughout the entire organism, and in harmony with 

 certain facts, we were bound to believe that digestion and as- 

 similation, or, to speak in other terms, the metabolic processes 

 of the various tissues, in a somewhat restricted sense, were 

 closely related. Beneath the common observation that '' diges- 

 tion waits on appetite " lies the deeper truth that food is not 

 prepared in the alimentary canal (digested) without some rela- 

 tion to the needs of the system generally. In other words, the 

 voice of the tissues elsewhere is heard in the councils of the 

 digestive tract, and is regarded ; and this is effected chiefly 

 through the nervous system. Excess in eating may lead to 

 vomiting or diarrhoea— plain ways of getting rid of what can 

 not be digested. 



Evolution, — ^We have already alluded to some of those modi- 

 fications in the form of the digestive organs that indicate an 

 unexpected plasticity, and impress the fact of the close rela- 

 tion of form and function. The conversion of a sea-gull into a 

 graminivorous bird, with a corresponding alteration in the na- 

 ture of the form of the stomach (it becoming a gizzard), with 

 doubtless modifications in the digestive processes, when re- 

 garded more closely, implies coadaptations of a very varied 

 kind. These are as yet but imperfectly known or understood, 

 and the subject is a wide and inviting field for the physiologist. 

 Darwin and others have indicated, though but imperfectly, 

 some of the changes that are to be regarded in animals as cor- 

 relations ; but in physiology the subject has received but little 

 attention as yet. We have in several parts of this work called 

 attention to it ; but the limits of space prevent us doing little 

 more than attempting to widen the student's field of vision by 

 introducing such considerations. The influence of climate on 

 metabolism, an undoubted fact, has many implications. 



Any one who keeps a few wild animals in confinement un- 

 der close observation, and endeavors to ascertain how their 

 natural, self-chosen diet may be varied when confined, will 

 be astonished at the plasticity of their instincts, usually con- 

 sidered as so rigid in regard to feeding. These facts help one 



