424 SCIENTIFIC THOUGHT. 



living matter, revealed by the most powerful microscope 

 aided by all the elaborate processes of staining, still 

 appear to be endowed with the fundamental properties 

 of life, such as irritability, contractility, and metabolism, 

 i.e., change in form and chemical constitution, the 

 object of this line of research, viz., the investigation 

 of the initial structure of the elements of living matter, 

 can only be reached by indirect means or by conjecture. 

 Structural chemistry and stereo-chemistry proceed by 

 similar methods of investigation, and have succeeded 

 by means of the atomic, molecular, and kinetic theories 

 of matter in bringing order and unity into a very large 

 portion of our knowledge of chemical combinations. The 

 morphological or structural biologist pictures to himself 

 very much more complicated arrangements of molecules 

 than the carbon tetrahedron of van 't Hoff or the benzine 

 ring of Kekule, yet formed on similar principles ; and 

 by continuing in his mind these combinations which, 

 as they become more complex, also become more un- 

 stable, he arrives ultimately at a very complex and 

 continually changing chemical structure, which he imag- 

 ines might be the beginning of the living process, the 

 element of organisation. This conception, so far as I 

 can find, was first introduced into biological literature 

 by Herbert Spencer. He has termed this element 

 of living matter " the physiological unit." The con- 

 ception has been varied in endless ways by many sub- 

 sequent biologists, all of whom have invented special 

 names for these elementary units of life out of which 

 they hope to put together the many observable proto- 

 plasmic and cellular tissues of the plant and animal 



