516 Theory of the Nutrition [Bookiii. 



ally, and in the doctrine of nutrition particularly, in the period 

 from 1758 to 1832, we have only to compare the contents of 

 these two books. That this progress was a considerable one, 

 appears plainly from a short summary at the end of the first 

 volume of the general theory of nutrition, as De Candolle him- 

 self conceived it ; this summary will show us at the same time 

 that he aimed rather at giving a clear account of the whole of 

 the internal economy of the plant, than at searching into the 

 moving forces, the causes and effects. From this he was 

 necessarily withheld by his assumption of a vital force. He 

 distinguished four kinds of forces; the force of attraction which 

 produces the physical, and that of elective affinity which causes 

 the chemical phenomena; then the vital force, the original 

 source of all physiological, and the soul-force, the cause of all 

 psychical phenomena. Only the first three of these forces 

 operate in the plant, and though it is necessary to find out what 

 phenomena in vegetation are due to physical or chemical causes, 

 yet the main task of the vegetable physiologist is to discern 

 those which proceed from the vital force, and the chief mark of 

 such phenomena is that they cease with the death of the plant 

 (p. 6). Of course therefore all the peculiar phenomena of nutri- 

 tion, which are manifested only in the living plant, come within 

 the domain of the vital force. It must be allowed, however, that 

 De Candolle has made a very moderate use of the vital force, 

 and confines himself wherever he can to physical and chemical 

 explanations ; and when he has recourse to the vital force, it is 

 owing less to the influence of his philosophical point of view 

 than to the fact that his account is based rather on tradition and 

 information at second hand than on actual research. It is true 

 that De Candolle was perhaps better acquainted than any con- 

 temporary botanist with the physics and chemistry of his day, 

 and it is part of his great merit that he should have acquired 

 so much knowledge on these subjects while engrossed in his 

 splendid labours as a systematist and morphologist ; but he be- 

 trays, at least in his later years, a want of practice in the study 



