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, i.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, i.e. 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 sensitive 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, i.e. 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 spon- 



