fect occur singly, or
combined? So far from comparison being in any way peculiar to Biological
science, it is, I think, the essence of every science.

A speculative philosopher again tells us that the Biological sciences
are distinguished by being sciences of observation and not of
experiment![5]

Of all the strange assertions into which speculation without practical
acquaintance with a subject may lead even an able man, I think this is
the very strangest. Physiology not an experimental science! Why, there
is not a function of a single organ in the body which has not been
determined wholly and solely by experiment. How did Harvey determine the
nature of the circulation, except by experiment? How did Sir Charles
Bell determine the functions of the roots of the spinal nerves, save by
experiment? How do we know the use of a nerve at all, except by
experiment? Nay, how do you know even that your eye is your seeing
apparatus, unless you make the experiment of shutting it; or that your
ear is your hearing apparatus, unless you close it up and thereby
discover that you become deaf?

It would really be much more true to say that Physiology is _the_
experimental science _par excellence_ of all sciences; that in which
there is least to be learnt by mere observation, and that which affords
the greatest field for the exercise of those faculties which
characterise the experimental philosopher. I confess, if any one were to
ask me for a model application of the logic of experiment, I should know
no better work to put into his hands than Bernard's late Researches on
the Functions of the Liver.[6]

Not to give this lecture a too controversial tone, however, I must only
advert to one more doctrine, held by a thinker of our own age and
country, whose opinions are worthy of all respect. It is, that the
Biological sciences differ from all others, inasmuch as in _them_
classification takes place by type and not by definition.[7]

It is said, in short, that a natural-history class is not capable of
being defined--that the class Rosaceae, for instance, or the class of
Fishes, is not accurately and absolutely definable, inasmuch as its
members will present exceptions to every possible definition; and that
the members of the class are united together only by the circumstance
that they are all more like some imaginary average rose or average fish,
than they resemble anything else.

But here, as before, I think the distinction has arisen e