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relations with its surroundings. Upon this subject Treviranus* 
expresses himself as follows : — 
iC Under the term ‘ life J we form for ourselves a conception 
of a condition of activity. We speak of an animal or a plant 
as being f living 3 so long as we can still detect in it signs 
of growth and movement, and therefore of activity. But at 
the same time we conceive of this activity as being something 
which originates in the body to which w T e ascribe life, from 
the interior and not ab extra. The sea, when disturbed by 
storms, also exhibits activity, but we do not for this reason 
ascribe c life 3 to it. We do not do this, because its every 
movement is the result of the application of an external force. 
Every movement, then, which originates in extraneous forces 
and is merely imparted to the body, w r e term a f mechanical 3 
movement ; and those movements which occur as the manifes- 
tations of life, are distinguished from those which are merely 
mechanical, by the fact that they find their starting-point in 
internal and not in external causes. Easy, however, as it 
may appear at first sight to separate mechanical from vital 
movements, nevertheless a closer examination shows us that 
the above-mentioned ground of distinction is an insufficient 
one. If the living body were an entirely isolated system, 
which contained in itself the source and spring of all its move- 
ments, then, certainly, it would be easy to draw the line 
between mechanical and vital movements. But, all the mani- 
festations of vital activity are products of an interaction 
between the living body and the outer world, and this is 
precisely the case also with mechanical movements. A mass 
which has been set in movement by an external impulse, does 
not less react against the impelling body, than does the 
muscle-fibre against the stimulus which calls forth its con- 
tractility. What, then, is the distinctive character between 
the interaction which gives rise to the mechanical movement, 
and that by which the vital movement is originated ? In this 
lies the first of the difficulties which we have to combat in 
forming a conception of life/'’ 
It is not necessary to pursue further the line of argument 
indicated above. For our present purpose, it is sufficient 
to assume that all bodies in a state of active vitality are 
characterised by their power of maintaining a stable condition 
in the face of the agencies whereby their substance is con- 
stantly being disintegrated; that they make certain active 
responses to external and internal stimuli, and have other than 
merely passive relations with the external world; and, lastly. 
* Biologie, vol. i., p. 16. 1802. 
