PHYSIOLOGICAL RESEARCH, PHYSIOLOGICAL REASONING. 145 
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 acritical logic. Such a statement as the above would 
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 experimerits 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. 
To attempt to generalize for the heart, the kidney, the liver, 
etc., when only the dog, cat, rabbit, and frog, have been made 
as arule the subjects of experiment, except for the groups of 
animals to which the above belong, is not only hazardous but 
positively illogical; while to denominate conclusions based on 
such experiments, even when supplemented by the teachings 
of disease, “human physiology” is, in the writer’s opinion, a 
wholly unwarrantable proceeding. 
It is this conviction which has had much to do with this 
book being written; to the introduction of the comparative 
element; and the separation so frequently in form as well as 
in reality of facts and inferences. A genuine human physi- 
ology, with the exact nature and value of the inferences clearly 
stated, is yet to be written; and it seems not only judicious, 
but demanded as a matter of candor and honesty, to state at 
the outset to the student what we feel able to teach confidently, 
and what must be presented as feebly probable or barely pos- 
sible. 
Human physiology proper must of necessity be accumulated 
slowly. Much may be, indeed must be, inferred from the ex- 
periments disease is making; still, certain forms of accident or 
surgical operation provide the opportunity to investigate the 
human body in health or in a moderately near approach to that 
condition. Close self-observation under a variety of condi- 
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