I have myself timed the heart of these little animals.  I found it as regular as possible in its periods of reversal:  and I know no spectacle in the animal kingdom more wonderful than that which it presents—­all the more wonderful that to this day it remains an unique fact, peculiar to this class among the whole animated world.  At the same time I know of no more striking case of the necessity of the verification of even those deductions which seem founded on the widest and safest inductions.

Such are the methods of Biology—­methods which are obviously identical with those of all other sciences, and therefore wholly incompetent to form the ground of any distinction between it and them.[8]

But I shall be asked at once, Do you mean to say that there is no difference between the habit of mind of a mathematician and that of a naturalist?  Do you imagine that Laplace might have been put into the Jardin des Plantes, and Cuvier into the Observatory, with equal advantage to the progress of the sciences they professed?

To which I would reply, that nothing could be further from my thoughts.  But different habits and various special tendencies of two sciences do not imply different methods.  The mountaineer and the man of the plains have very different habits of progression, and each would be at a loss in the other’s place; but the method of progression, by putting one leg before the other, is the same in each case.  Every step of each is a combination of a lift and a push; but the mountaineer lifts more and the lowlander pushes more.  And I think the case of two sciences resembles this.

I do not question for a moment, that while the Mathematician is busied with deductions from general propositions, the Biologist is more especially occupied with observation, comparison, and those processes which lead to general propositions.  All I wish to insist upon is, that this difference depends not on any fundamental distinction in the sciences themselves, but on the accidents of their subject-matter, of their relative complexity, and consequent relative perfection.

The Mathematician deals with two properties of objects only, number and extension, and all the inductions he wants have been formed and finished ages ago.  He is occupied now with nothing but deduction and verification.

The Biologist deals with a vast number of properties of objects, and his inductions will not be completed, I fear, for ages to come; but when they are, his science will be as deductive and as exact as the Mathematics themselves.