THE VITAL IMPETUS 127 



retard physiological discovery was the lack of progress 

 made by chemistry and physics, and this may have 

 been the result of the Stahlian phlogistic hypothesis. 



However this may be, it seems clear that it was 

 the discoveries of the great chemists of the close of the 

 eighteenth century that again introduced mechanistic 

 views into physiology. With the discoveries of 

 Lavoisier and his successors the latter science ac- 

 quired new methods of research and the older working 

 hypotheses were re-introduced. There has been no 

 recession from this position during the nineteenth 

 century. Mechanistic biology culminated in the 

 writings of Huxley and Max Verworn and received a 

 new accession of strength almost in our own day in 

 the modern discoveries of physical chemistry ; and 

 when physiology became truly a comparative science, 

 and embraced the lower invertebrates, it became 

 perhaps most mechanistic — witness the writings of 

 Jacques Loeb. 



Of far greater philosophical importance than the 

 physico-chemical investigation of the functioning of 

 individual organisms has been the essentially modern 

 experimental study of embryological processes. The 

 former deals essentially with the means of growth, 

 reproduction, and so on. We can no longer doubt 

 that the changes which we can observe taking place 

 in the organism, either the developing embryo or the 

 fully formed animal, are, in the long run, physico- 

 chemical changes ; and in ultimate analysis we cannot 

 expect to find anything else than processes of this 

 nature. 



But physiological investigation has failed to 

 provide anything more than this analysis. If we 

 watch the development of the egg of an animal into a 

 larval form, and continue to trace the metamorphosis 



