420 SCIENTIFIC THOUGHT. 



formation, growth, division, and maintenance of form 

 amidst change of matter and alternation of function — 

 constitutes the " prolegomena " of physiology, and a com- 

 parison of Prof. 0. Hertwig's recent publication on the 

 " cell " with the introduction to Johannes Mliller's ' Phys- 

 iology ' marks well the change of ideas which half a 

 century has produced. And we must so much the more 

 admire the clear anticipation of Schwann, as he was not 

 in possession of the fvill conception of energy in its 

 two interchangeable forms of kinetic and potential 

 energy, which was developed in the course of the two 

 decades following his publication. Schwann not only 

 conceived the cell to be the morphological unit of all 

 living matter, but he also saw that " cell formation 

 must be the general principle of organic development, 

 and that there can be only one such principle." In 

 the third section of his ' Microscopical Eesearches ' he 

 founds on this " his theory of organisms, and rejects 

 distinctly therein all teleological explanations based upon 

 a vital force acting according to final purposes." He 

 thus showed " that the only essential property of all 

 living matter — viz., growth — is not inaccessible to a 

 physical explanation," and he did this at a time " when 

 Liebig had not yet taught physiologists the chemical 

 changes which take place in living tissues." These ideas 

 were only partially adopted by Johannes Miiller and 

 other leading physiologists of the day. Schwann's view 

 could only be consistently elaborated in proportion as 

 Circulation to the older conception of a " Stoffwechsel " (a chem- 

 andmergy. ic^l proccss) there was added that of a " Kraft "- or 



