H OF THE NATURAL HISTORY SCIENCES 55 



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 generalisations 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 ci 

 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 invari- 

 able 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, 



C>4 



