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manifested by lining matter, we may next consider what we 
actually mean when we speak of “life” or “ vitality;” and 
this, though apparently a simple matter, is really one about 
which considerable differences of opinion have prevailed. As 
for definitions of life, many such have been framed by eminent 
philosophers, and would be quoted if any advantage were 
derivable therefrom. As an old definition we may take that 
of Treviranus, who defines life as “ the constant uniformity of 
phenomena with diversity of external influences.” As a 
modern definition that of Mr. Herbert Spencer may be 
selected, who considers life to be “the definite combination of 
heterogeneous changes, both simultaneous and successive, in 
correspondence with external coexistences and sequences.” 
When we come, however, to examine these and other short 
definitions of life, we shall generally find that they practically 
amount to stating, in terse phraseology, that life or vitality is 
the sum of the phenomena manifested by a living being ; in 
other words, that life is life. It appears to me, however, that 
there is, or ought to be, in any satisfactory definition of life, 
the underlying conception that life is something more than 
the mere sum of the 'phenomena of a living being ; and that it 
is, in fact, the force or aggregate of forces to the operation of 
which these phenomena are due. Life, in fact, is the force or 
group of forces in virtue of which protoplasmic matter, under 
given conditions, passes through a succession of changes, 
which correspond with, and are determined by, internal and 
external impressions, and follow one another in a more or less 
definite and determinable sequence. 
Leaving, however, short definitions on one side, we may see 
what more particularly are those phenomena of living beings 
in virtue of which we ascribe to them the possession of “life,” 
and distinguish them fundamentally from all forms of dead 
matter. So far as this point is concerned, it is at once evident 
that a living body is only thereby distinguishable from a dead 
one that it performs the three great physiological functions 
previously enumerated; or that, at any rate, it has the power of 
discharging these functions under appropriate conditions and 
stimuli. Every living body nourishes itself, and maintains its 
existence as an individual, in spite of the constant destruction 
of the matter of which it is composed. Every living body has 
certain active relations with the external world. Every living 
body can reproduce itself. It cannot be doubted, however, 
that the most striking phenomena of a living body, or at least 
those which are most obviously opposed to what we see in 
merely dead matter, are those which arise from its having active 
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