2 NERVOUS SYSTEM AND SENSE-ORGANS 
place the food taken in must obviously be larger in amount 
than when it is merely a question of compensating for waste. 
By a process of over-growth with subsequent separation from 
the parent-body new individuals are developed, capable of leading 
independent existences, and ultimately giving rise to a further 
generation in their turn. Another very characteristic property 
of protoplasm is Contractility, z.e. spontaneous change of shape. 
Hence all the various kinds of Animal Movement, without 
which food could not be secured, enemies escaped, or unfavour- 
able surroundings quitted. 
The present section is an expansion of the last part of the 
brief sketch of Human Physiology already mentioned, ze. the 
part headed Nervous System and Sense Organs. What these 
are, and why they should exist, cannot be understood without 
reference to another fundamental property of protoplasm, which 
we may broadly term Sensitiveness and Spontaneity, there being, 
unfortunately, no briefer way of putting it. The surroundings 
of an animal are constantly changing; all sorts of external agents 
are continually acting upon it to varying extents; and life wholly 
depends upon successful adjustment or adaptation to this per- 
petually altering Environment. Alternations of day and night, 
succession of seasons, tidal flow and ebb, variations of food- 
supply, the diminution or increase in number of enemies, may 
be taken as examples of changes which have much to do with 
the preservation or extinction of old species and the evolution 
of new ones. That protoplasm is sezsztzve means that it is not 
inert to its surroundings, but reacts, in ways which tend to the 
preservation of life, to the influences which are constantly affect- 
ing it. If, when you are not looking, someone touches your 
hand with a red-hot poker, the member thus treated is drawn 
back without the exercise of will-power, and immediately after 
a painful sensation is experienced. This practically illustrates 
the fact that human protoplasm is sensitive to one external 
agent, ze. heat, and the usefulness of reaction is sufficiently 
obvious. If animals were not sensitive to heat many of them 
would very quickly perish in an untimely manner. And a 
little consideration will make it apparent that Sensitiveness to 
a great variety of external agents is absolutely necessary to 
existence. All actions, however, are not the direct results of 
external agents acting for the time being. Protoplasm is sfon- 
