taken for
granted, that there is a very wide difference between the Physiological
and other sciences in point of method.

In the first place it is said--and I take this point first, because the
imputation is too frequently admitted by Physiologists themselves--that
Biology differs from the Physico-chemical and Mathematical sciences in
being "inexact."

Now, this phrase "inexact" must refer either to the _methods_ or to the
_results_ of Physiological science.

It cannot be correct to apply it to the methods; for, as I hope to show
you by and by, these are identical in all sciences, and whatever is true
of Physiological method is true of Physical and Mathematical method.

Is it then the _results_ of Biological science which are "inexact"? I
think not. If I say that respiration is performed by the lungs; that
digestion is effected in the stomach; that the eye is the organ of
sight; that the jaws of a vertebrated animal never open sideways, but
always up and down; while those of an annulose animal always open
sideways, and never up and down--I am enumerating propositions which are
as exact as anything in Euclid. How then has this notion of the
inexactness of Biological science come about? I believe from two causes:
first, because, in consequence of the great complexity of the science
and the multitude of interfering conditions, we are very often only
enabled to predict approximatively what will occur under given
circumstances; and secondly, because, on account of the comparative
youth of the Physiological sciences, a great many of their laws are
still imperfectly worked out. But, in an educational point of view, it
is most important to distinguish between the essence of a science and
the accidents which surround it; and essentially, the methods and
results of Physiology are as exact as those of Physics or Mathematics.

It is said that the Physiological method is especially _comparative_[4];
and this dictum also finds favour in the eyes of many. I should be
sorry to suggest that the speculators on scientific classification have
been misled by the accident of the name of one leading branch of
Biology--_Comparative Anatomy_; but I would ask whether _comparison_,
and that classification which is the result of comparison, are not the
essence of every science whatsoever? How is it possible to discover a
relation of cause and effect of _any_ kind without comparing a series of
cases together in which the supposed cause and ef