IX THE HISTORY AND SCOPE OF ZOOLOGY 361 



the various lines of scientific progress, Priestley in 

 his discovery of oxygen was as much concerned 

 in the study of a chlorophyll -bearing Protozoon, 

 Euglena viridis, as in that of the red oxide of 

 mercury ; and the interest in " vital spirits " as a 

 physiological factor was an important stimulus to 

 those researches which produced modern chemical 

 knowledge. 



The purely anatomical side of physiological pro- 

 gress is marked in the beginning of the nineteenth 

 century by the work of Bichat (1771-1802), who distin- 

 guished by naked-eye characters the different struc- 

 tural materials of which the organs of man and the 

 higher animals are built, and thus founded in first out- 

 line the science of Histology. By the end of the first 

 quarter of this century it had become clear to the 

 minds of the anatomico- physiological students of 

 animal life that the animal body was subject to the 

 same physical laws as other matter, although it was 

 still held that some additional and mysterious agent 

 so-called " vitality " was at work in living bodies. 

 It had become clear that animal material could be 

 investigated chemically, and that the processes of 

 digestion, assimilation, respiration, and secretion were 

 chemical processes. 



To a considerable extent the chemical composition 

 and properties of the tissues, and the chemical nature 

 of the various changes of life and of putrefaction after 

 death, had been investigated, but one step was yet to 

 be taken which brings the study of ultimate structure, 

 chemical activity,, form, and the development of form 



