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spite, however, of tlie excessively unstable chemical equili- 
brium of living protoplasm, the living body is enabled to 
maintain its stability and its average condition for a longer or 
shorter period, which is normally constant, or approximately 
so, for each organism. This is due to the fact that every 
mass of living protoplasm has the power of taking in from the 
exterior certain foreign materials, and of f f assimilating 99 these, 
or of converting these into new protoplasm capable of re- 
placing the loss and destruction of this substance due to the 
act of living. In this connection, however, we must notice 
the remarkable fact that the assimilative powers of vegetable 
and animal protoplasm differ in a very remarkable manner. 
Thus the protoplasm of plants, when associated with the green 
colouring-matter known as “ chlorophyll/’ has the power of de- 
composing carbon dioxide in the presence of sunlight ; and all 
plants possess the power of building up new protoplasm out 
of the inorganic substances, ammonia, carbon dioxide, and 
water. On the other hand, no known animal can thus syn- 
thetically construct protoplasm out of merely inorganic ma- 
terials, but all require to be supplied with ready-made proto- 
plasm as food. The true significance of this fact cannot, with 
our present knowledge, be rightly estimated ; but it is de- 
serving of especial notice, as showing that though the proto- 
plasm of animals and that of plants are, so far as we know, 
structurally and chemically identical, there nevertheless exists 
a vast and most important difference in the vital functions 
which each is able to discharge in the economy. 
2. All masses of living protoplasm, whether these are inde- 
pendent organisms, or form parts of an organic whole, have 
certain active relations with their surroundings. This is espe- 
cially seen in the power of movement exhibited by protoplasm 
in all its forms (when free to move) in response to certain 
external or internal stimuli. This “ irritability,” or the power 
of actively responding to stimulation, is a feature entirely 
confined to living protoplasm, and is not exhibited by any of 
the multifarious forms of dead matter, all of which ar q passive, 
and merely exhibit appropriate reactions when influenced by 
external forces. 
3. Lastly, living protoplasm has the unique power of repro- 
ducing itself by the process of detaching a portion of its own 
substance, which, under suitable conditions, will become 
developed into a new living being, resembling that by which 
it was originally given off. 
Having now briefly glanced at the principal phenomena 
