4 THE ACTION OF THE LIVING CELL 



mon in the biological sciences. Frequently, as we shall 

 see, we are unable definitely to isolate and characterize 

 the nature of a physiologically active substance, al- 

 though empirical experimental facts demonstrate the 

 presence of such substances and the probable role 

 which they play in the activity of the living organism. 

 As the American physicist Millikan (1931) remarks, 

 "The distinguishing feature of modern thought lies 

 in the fact that it begins by discarding all a priori con- 

 ceptions concerning the nature of reality — and takes 

 as its starting point experimental facts whether they 

 fit into any general philosophical scheme or not. — In 

 a word, modern science is essentially empirical." 



Galileo's simple experiment on the velocity of fall- 

 ing bodies of dissimilar mass sounded the death knell 

 of an experimentally unsupported philosophical be- 

 lief, and may be regarded as the birth of experimental 

 science. In the biological sciences, both pure and 

 applied, similar unsupported hypotheses exist even 

 today, but these are one by one being subjected to the 

 critical test of experiment. Indeed, at present experi- 

 mental biology is far in advance of any adequate the- 

 oretical background. 



In a colony of cells we must consider the presence 

 of all members of the colony as environmental factors 

 of any single cell under consideration. In such a 

 colony the activities, growth and reproduction (which 

 may conveniently be regarded as a special aspect of 

 growth) of individual cells is conditioned by the 

 presence of other cells, both homologous and hetero- 

 geneous. For example, the growth of yeast cells is 

 more rapid in cultures heavily seeded with the inocu- 

 lum than in cultures started from a few cells. In 1901 



