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 entirely from confusing a transitory imperfection with an essential character.  So long as our information concerning them is imperfect, we class all objects together according to resemblances which we feel, but cannot define:  we group them round types, in short.  Thus, if you ask an ordinary person what kinds of animals there are, he will probably say, beasts, birds, reptiles, fishes, insects, &c.  Ask him to define a beast from a reptile, and he cannot do it; but he says, things like a cow or a horse are beasts, and things like a frog or a lizard are reptiles.  You see he does class by type, and not by definition.  But how does this classification differ from that of the scientific Zoologist?  How does the meaning of the scientific class-name of “Mammalia” differ from the unscientific of “Beasts”?