348 THE BIOLOGICAL SCIENCES AND MEDICINE. 



mechanical explanation of its contraction by the influx 

 of animal spirits. 



The discoveries of Trembley tended in the same di- 

 rection. 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 was 

 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 or- 

 ganisation (as then understood), sapped the foundations 

 of the Cartesian physiology as a complete expression of 

 vital phenomena. 



For Wolff, the physical basis of life is a fluid, pos- 

 sessed of a " vis essentialis " and a " solidescibilitas," in 

 virtue of which it gives rise to organisation ; and, as he 

 points out, this conclusion strikes at the root of the whole 

 iatro-mechanical system. 



In this country, the great authority of John Hunter 

 exerted a similar influence ; though it must be admitted 

 that the too sibylline utterances which are the outcome 

 of Hunter's struggles to define his conceptions are often 

 susceptible of more than one interpretation. Neverthe- 

 less, on some points Hunter is clear enough. For exam- 

 ple, he is of opinion that " Spirit is only a property of 

 * " Tbeoria Gencrationis," 1759. 



