T.] VALUE OF NATURAL HISTORY SCIENCES. 73 



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 sub 

 ject 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 d 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 



