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.

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