Philosophical Conceptions of Life. 250 
higher animals. Their sensibility to the influence of the 
environment is manifested in several ways. They grow, that 
is they appropriate materials from the environment, in the 
way I have already specified ; they manifest automatic move- 
ments, that is, on encountering food, obstacles, or other dis- 
turbing external circumstances, movements result the direction 
and energy of which are in no wise determined by the cha- 
racter or force of the external influences, or, as they may be 
conveniently termed, the stimuli by which these movements 
are provoked; and finally, simultaneously with the process 
of growth, a certain metamorphosis, or metabolism, of the 
protoplasm is continually going on, resulting in the formation ~ 
of excrementitious substances which are continually being 
excreted. 
The processes of growth and metabolism exhibit different 
degrees of intensity in accordance with variations of the envi- 
ronment ; and whatever physical theory of the mode in which 
the protoplasmic motions are produced we may adopt, the 
mechanical force manifested can only be supposed to proceed 
from the decomposition of a part of the protoplasm itself into 
simpler compounds, that is, from a particular kind of meta- 
bolism. Hence you will, I think, be quite prepared to hear 
me speak of all the circumstances in the environment that so 
act upon living protoplasm as to increase its growth or meta- 
bolism as stimuli, and of the property of living protoplasm 
by which all its responses to stimuli are guided as irrita- 
bility, instead of limiting these terms to the phenomena of 
automatic movement only, as was formerly done. This irri- 
tability of living protoplasm determines the direction in which 
its internal forces shall be manifested. Speaking of it as I 
do, perhaps you would wish me to call it sensibility rather 
than irritability ; and I do not know that I should object very 
strenuously to any one who wished to do this. But however 
you mayname it, it is this vital property of all living protoplasm 
that produces the sensibility to changes in the environment, 
which has been the main factor in the gradual evolution, durmg 
the ages, of the highest and most complex from the simplest 
and lowest living forms. 
Against this view it has been urged with much ingenuity 
that protoplasm is the material substratum of life, and life 
merely a property of protoplasm; that is, if the words have 
any meaning at all, that life is the resultant only of the forces 
inherent in the inorganic atoms of which the protoplasm is 
built up. Now, in the first place, no one has ever yet been 
able to show, by any conceivable synthesis, how the forces 
known to belong to the several kinds of inorganic atoms, of 
