v.] EDUCATIONAL VALUE OF NATURAL UISTORY SCIENCES. 85 



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 ex- 

 . istence 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 

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