XIV BIOLOGICAL SCIENCES AND MEDICINE 363 



central source of power, and the parts of the 

 machine are merely passive distributors of that 

 power. The Cartesian school conceived of the 

 living body as a machine of this kind; and herein 

 they might have learned from Galen, who, what- 

 ever ill use he may have made of the doctrine of 

 " natural faculties,^^ nevertheless had the great 

 merit of perceiving that local forces play a great 

 part in physiology. 



The same truth was recognised by Glisson, but 

 it was first prominently brought forward in the 

 Hallerian doctrine of the " vis insita " of muscles. 

 If muscle can contract without nerve, there is an 

 end of the Cartesian mechanical explanation of 

 its contraction by the influx of animal spirits. 



The discoveries of Trembley tended in the 

 same direction. In the freshwater Hydra, no 

 trace was to be found of that complicated 

 machinery upon which the performance of the 

 functions in the higher animals were supposed to 

 depend. And yet the hydra moved, fed, grew, 

 multiplied, and its fragments exhibited all the 

 powers of the whole. And, finally, the work of 

 Caspar F. Wolff,* by demonstrating the fact that 

 the growth and development of both plants and 

 animals take place antecedently to the existence 

 of their grosser organs, and are, in fact, the causes 

 and not the consequences of organization (as 

 then understood), sapped the foundations of the 



* Theoria Generationis, 1759. 



