146 THE SCIENCE OF BIOLOGY 



ary problem from the side of adult organization rather than 

 from the side of individual development. But the fact is 

 acknowledged that evolutionary origins must, in the last 

 analysis, be explained as changes originating within cells 

 and perpetuated by a cellular mechanism. 



RAMIFICATIONS OF THE CELL-THEORY 



Within the field of physiological science, as well as in 

 zoology, applications of the cell-theory were the great 

 achievement of the nineteenth century. When bodily 

 activities were seen to be nothing but cell activities, the 

 way was opened for a comprehensive explanation of all 

 general functions in both animals and plants. Take the 

 production of a secretion such as saliva, gastric juice, or 

 perspiration: The older physiologists had studied the 

 phenomena which were visible to the unaided eye. The 

 nature of secretions had been investigated, so far as chemi- 

 cal knowledge allowed. Many interesting phenomena had 

 been ascertained. 9 There remained the problem of how 

 the chemical compounds dissolved in the water of a partic- 

 ular secretion were produced and secreted. The series of 

 chemical changes involved in the process is still incompletely 

 known. The setting can, however, be described in terms 

 of cells. 



One of the glands in the skin of a frog will illustrate what 

 happens in cases where the secretion is passed to the outside 

 or to an internal cavity like the stomach. The gland in 

 this instance is relatively simple, consisting of a flask- 



9 A classical example is the work of the American physician, William Beau- 

 mont, who was fortunate in having under observation, for some years following 

 1822, a soldier who had been wounded in the stomach and whose wound had 

 healed in such a manner that an orifice remained through which the processes 

 of secretion and digestion could be observed. Many problems of gastric 

 digestion are still investigated, in a fashion comparable with that pursued by 

 Beaumont, without reference to the origin of the gastric juice within the cells, 

 but the formation and liberation of any secretion is a cellular function. 



