100 ORGANISM AND ENVIRONMENT 
seem to pass through an organism are constantly being 
replaced. Nor is it mere form: for the flowing mate- 
rial is intensely specific. Structure, composition and 
activity are inseparably blended together in life, and 
no phenomenon in the inorganic world seems to us to 
be similar to the phenomenon of life. The funda- 
mental facts with regard to life do not fit into the 
conceptions by means of which we at present interpret 
inorganic phenomena. Life is something which the 
biologist as such must treat as a primary reality, and 
no mere artifact. It is with life, and not merely with 
physics or chemistry, or bio-physics or bio-chemistry, 
that these lectures have dealt. From the outset of my 
own scientific work I have been guided by the concep- 
tion that it is with life, and not with what physics and 
chemistry are at present capable of interpreting, that 
physiology deals; and this conception has grown 
clearer in my mind as a scientific working hypothesis 
with advancing experience as a physiological worker. 
What aims does this conception carry with it for 
physiological investigation? The ground hypothesis 
or conception is that each detail of organic structure, 
composition, and activity is a manifestation or expres- 
sion of the life of the organism regarded as a separate 
and persistent whole. We have therefore to make 
use of this hypothesis as a tool for investigation, just 
as the physicist uses the conceptions of mass and 
energy, or the chemist the atomic theory. We assume, 
therefore, that it will be found on sufficient investiga- 
tion that the scattered observations of living organisms 
with which preliminary sensory observations supply 
