THE CIRCULATION OF THE BLOOD. 451 



reference of them to distinct ideas of causation, their interpretation as 

 the results of mechanical force, was omitted or attempted in vain. The 

 very notion of such Force, and of the manner in which motions were 

 determined by it, was in the highest degree vague and vacillating; and 

 a century was requisite, as we have seen, to give to the notion that 

 clearness and fixity which made the Mechanics of the Heavens a pos- 

 sible science. In like manner, the notion of Life, and of Vital Forces, 

 is still too obscure to be steadily held. We cannot connect it distinctly 

 with severe inductions from facts. We can trace the motions of the 

 animal fluids as Kepler traced the motions of the planets ; but when 

 we seek to render a reason for these motions, like him, we recur to 

 terms of a wide and profound, but mysterious import ; to Virtues, In- 

 fluences, undefined Powers. Yet we are not on this account to despair. 

 The very instance to which I am referring shows us how rich is the 

 promise of the future. Why, says Cuvier, 18 may not Natural History 

 one day have its Newton ? The idea of the vital forces may gradually 

 become so clear and definite as to be available in science ; and future 

 generations may include, in their physiology, propositions elevated as 

 far above the circulation of the blood, as the doctrine of universal gravi- 

 tation goes beyond the explanation of the heavenly motions by epicy- 

 cles. 



If, by what has been said, I have exemplified sufficiently the nature 

 of those steps in physiology, which, like the discovery of the Circulation, 

 give an explanation of the process of some of the animal functions, it is 

 not necessary for me to dwell longer on the subject ; for to write a his- 

 tory, or even a sketch of the history of Physiology, would suit neither 

 my powers nor my purpose. Some further analysis of the general 

 views which have been promulgated by the most eminent physiologists, 

 may perhaps be attempted in treating of the Philosophy of Inductive 

 Science ; but the estimation of the value of recent speculations and in- 

 vestigations must be left to those who have made this vast subject the 

 study of their lives. A few brief notices may, however, be here intro- 

 duced. 



19 Ossem. Foss. Introd. 



