162 COMPARATIVE PHYSIOLOGY. 



and function, and in so far is the above generalization probable. 

 Such, a statement would, however, be far from that degree of 

 probability which is possible, and should therefore not be ac- 

 cepted till more evidence has been gathered. The mere resem- 

 blance in form and general function does not suffice to meet the 

 demands of a critical logic. Such a statement as the above woidd 

 not necessarily apply to the hearts of all vertebrates or even all 

 rabbits, if the experiments had been conducted on one animal 

 alone, for the result might be owing to a mere idiosyncrasy of 

 the rabbit under observation. The further we depart from the 

 group of animals to which the creature under experiment be- 

 longs, the less is the probability that our generalizations for 

 the one class will apply to another. It will, therefore, be seen 

 that wide generalizations can not be made with that amount of 

 certainty which is attainable until experiments shall have be- 

 come very numerous and widely extended. A really broad and 

 sound physiology can only be constructed when this science 

 has become much more comparative — that is, extended to many 

 more groups and sub-groups of animals than at present. 



We have incidently alluded throughout the work to the 

 teaching of disease. " Disease " is but a name for disordered 

 function. One viewing a piece of machinery for the first time 

 in improper action might draw conclusions with comparative 

 safety, provided he had a knowledge of the correct action of 

 similar machines. Our experience gives us a certain knowl- 

 edge of the functions of our own bodies. By ordinary observa- 

 _ tion and by experiment on other animals we get additional 

 data, which, taken with the disordered action resulting from 

 gross or molecular injury (disease), gives a basis for certain 

 conclusions as to the normal functions of the human body or 

 those of lower animals. This information is especially valu- 

 able in the case of man, since he can report with a fair de- 

 gree of reliability, in most diseased conditions, his own sensa- 

 tions. 



It is hoped that this brief treatment of the methods and 

 logic of physiology will suffice for the present. Throughout 

 the work they will be illustrated in every chapter, though not 

 always with distinct references to the nature of the intellectual 

 process followed. 



Summary. — There are two methods of physiological observa- 

 tion, the direct and the indirect. The flist is the simplest, and 

 •is valuable in proportion to the accuracy and delicacy and 



