PREFACE. 
Tue comparative method, the introduction of the teach- 
ings of embryology and of the welding principles of evolution 
as part of the essential structure of zodlogy, may be said to 
have completely revolutionized that science; and there is 
scarcely a text-book treating of the subject, however element- 
ary, which has not been molded in accordance with these 
guiding lines of thought. So far as I am aware, this can 
not be said of a single book on the subject of physiology. 
Feeling, therefore, that the time had come for the appearance 
of a work which should attempt to do, in some degree at 
least, for physiology what has been so well done for morphol- 
ogy, the present task was undertaken. But there were other 
changes which it seemed desirable to make. I think any one 
who will examine the methods and reasoning of the physi- 
ology of the day will not fail, on close scrutiny, to recognize 
a tendency to speak of certain conclusions, for various organs 
(and functions), as though they applied to these organs in 
whatever group of animals found, or, at all events, for man, 
no matter what the species of the animal that had been ex- 
perimented upon. For some years I have, in publications of 
my own original researches, strongly protested against such 
methods as illogical, I am wholly at a loss to understand 
how a work, built upon the most fragmentary and hetero- 
geneous evidence, derived from experiments on a few groups 
of animals, or a certain amount of human clinical or patho- 
logical evidence, can be fittingly termed a treatise on “ human 
physiology.” It will scarcely be denied that conclusions such 
