6o Resemblances and Differences Among Living Things 



ness to a great variety of activities that seem to be going on 

 outside the organism. Every organism is sensitive to such 

 activities and changes though not all to the same degree. 

 This sensitiveness we discover through observation and ex- 

 periment to lie in the constitution of the living matter itself, 

 and is sometimes spoken of as irritability. Plants are irrita- 

 ble as well as animals, as we may find from close observation, 

 although not all living things are equally sensitive to the same 

 disturbance, and although they do not all respond in the same 

 way when they are disturbed. 



Any one of the larger animals with which we are fa- 

 miliar is likely to respond to disturbance by some kind of 

 movement. When you approach a bird that is feeding peace- 

 fully on the ground, the bird takes flight before you have 

 time to reach it. If you pull an animal's tail it may perform 

 a variety of movements, some of which may amuse you or 

 worse, and some of which may bring the animal out of your 

 reach. Even a young baby performs a violent movement 

 — that is, a contraction of muscles — when something very 

 warm touches his skin. A sudden illumination brings about 

 muscular contraction around the pupil of the eye. The 

 sudden introduction of something ** sour " brings about vio- 

 lent contractions in the muscles of the face. In former times 

 it was well known that a relatively light touch on the flank 

 of a horse might start rapid contractions in the animal's legs. 



Movement such as we can see directly is not the only 

 means by which a living thing can show its irritability. In 

 our own bodies, for example, the taste of " sour " need not 

 produce any visible movements but does produce chemical 

 actions that show themselves in a flow of saliva. In the more 

 familiar animals there seems to be a greater degree of irritabil- 

 ity in certain limited regions which we sometimes call special 

 sense organs — the eye, the ear, the tongue, the lining of the 

 nose, and so on. The high degree of irritability in these or- 

 ganisms is associated with the presence of a special kind of 

 living tissue which we call nervous tissue. It has not been 

 possible so far to find anything in plants corresponding to 



