Why, exactly because the former depends on a definition, the latter on a type.  The class Mammalia is scientifically defined as “all animals which have a vertebrated skeleton and suckle their young.”  Here is no reference to type, but a definition rigorous enough for a geometrician.  And such is the character which every scientific naturalist recognises as that to which his classes must aspire—­knowing, as he does, that classification by type is simply an acknowledgment of ignorance and a temporary device.

So much in the way of negative argument as against the reputed differences, between Biological and other methods.  No such differences, I believe, really exist.  The subject-matter of Biological science is different from that of other sciences, but the methods of all are identical; and these methods are—­

1. Observation of facts—­including under this head that artificial observation which is called experiment.

2.  That process of tying up similar facts into bundles, ticketed and ready for use, which is called Comparison and Classification,—­the results of the process, the ticketed bundles, being named General propositions.

3. Deduction, which takes us from the general proposition to facts again—­teaches us, if I may so say, to anticipate from the ticket what is inside the bundle.  And finally—­

4. Verification, which is the process of ascertaining whether, in point of fact, our anticipation is a correct one.

Such are the methods of all science whatsoever; but perhaps you will permit me to give you an illustration of their employment in the science of Life; and I will take as a special case, the establishment of the doctrine of the Circulation of the Blood.

In this case, simple observation yields us a knowledge of the existence of the blood from some accidental haemorrhage, we will say:  we may even grant that it informs us of the localization of this blood in particular vessels, the heart, &c., from some accidental cut or the like.  It teaches also the existence of a pulse in various parts of the body, and acquaints us with the structure of the heart and vessels.

Here, however, simple observation stops, and we must have recourse to experiment.

You tie a vein, and you find that the blood accumulates on the side of the ligature opposite the heart.  You tie an artery, and you find that the blood accumulates on the side near the heart.  Open the chest, and you see the heart contracting with great force.  Make openings into its principal cavities, and you will find that all the blood flows out, and no more pressure is exerted on either side of the arterial or venous ligature.

Now all these facts, taken together, constitute the evidence that the blood is propelled by the heart through the arteries, and returns by the veins—­that, in short, the blood circulates.