Suppose our experiments and observations have been made on horses, then we group and ticket them into a general proposition, thus:—­all horses have a circulation of their blood.

Henceforward a horse is a sort of indication or label, telling us where we shall find a peculiar series of phaenomena called the circulation of the blood.

Here is our general proposition then.

How and when are we justified in making our next step—­a deduction from it?

Suppose our physiologist, whose experience is limited to horses, meets with a zebra for the first time,—­will he suppose that this generalization holds good for zebras also?

That depends very much on his turn of mind.  But we will suppose him to be a bold man.  He will say, “The zebra is certainly not a horse, but it is very like one,—­so like, that it must be the ‘ticket’ or mark of a blood-circulation also; and, I conclude that the zebra has a circulation.”

That is a deduction, a very fair deduction, but by no means to be considered scientifically secure.  This last quality in fact can only be given by verification—­that is, by making a zebra the subject of all the experiments performed on the horse.  Of course, in the present case, the deduction would be confirmed by this process of verification, and the result would be, not merely a positive widening of knowledge, but a fair increase of confidence in the truth of one’s generalizations in other cases.

Thus, having settled the point in the zebra and horse, our philosopher would have great confidence in the existence of a circulation in the ass.  Nay, I fancy most persons would excuse him, if in this case he did not take the trouble to go through the process of verification at all; and it would not be without a parallel in the history of the human mind, if our imaginary physiologist now maintained that he was acquainted with asinine circulation a priori.

However, if I might impress any caution upon your minds, it is, the utterly conditional nature of all our knowledge,—­the danger of neglecting the process of verification under any circumstances; and the film upon which we rest, the moment our deductions carry us beyond the reach of this great process of verification.  There is no better instance of this than is afforded by the history of our knowledge of the circulation of the blood in the animal kingdom until the year 1824.  In every animal possessing a circulation at all, which had been observed up to that time, the current of the blood was known to take one definite and invariable direction.  Now, there is a class of animals called Ascidians, which possess a heart and a circulation, and up to the period of which I speak, no one would have dreamt of questioning the propriety of the deduction, that these creatures have a circulation in one direction; nor would any one have thought it worth while to verify the point.  But, in that year, M. von Hasselt happening to examine a transparent animal of this class, found to his infinite surprise, that after the heart had beat a certain number of times, it stopped, and then began beating the opposite way—­so as to reverse the course of the current, which returned by and by to its original direction.