
INAUGURAL ADDRESS. OT 
more concisely in the words of Fletcher:* ‘‘ Life is the sum 
of the actions of organised beings, resulting directly from 
their vitality acted on by” [pabulum, conditions and 
stimuli}. Observe the word directly, for it is this which 
separates the non-vital from the vital functions, and makes 
this a definition of life in the abstract; vitality and its 
anatomical seat having been previously defined. With the 
same proviso of a previous explanation of the nature of 
protoplasm, I think the definition may be further simplified 
as follows: ‘Life is the interaction of protoplasm with 
the environment.” 
Now if this is a correct description of the cardinal fact 
in every vital act, it is not only a complete definition of 
life in the abstract, but it may be held to be a sufficient 
explanation of the nature of life to mark off biology to its 
place in the circle of the physical sciences in the wide sense. 
For, after all, what is an explanation? The explanation 
of any fact in nature is simply showing that it is the 
necessary outcome of other facts and laws known to us 
already in nature by observation and experience. And 
when we push the same question to these last and beyond 
them we very soon come to a point where no answer can 
be given. Nevertheless within certain limits the reference 
of any fact to a wider series of facts and laws is to all 
intents and purposes a sufficient explanation. I have 
given above the example of Newton in explaining the 
motions and positions of the heavenly bodies by the 
hypothesis of the extension of the terrestrial law of gravi- 
tation to all space. As this has been found to fit all the 
facts, it 1s universally received as a true explanation, 
although the physical cause of gravity was not discovered 
by Newton, and remains undiscovered still. In like 
manner may we not consider it a sufficient explanation 
* Rudiments of Physiology, p. 34. 
