THE BEHAVIOUR OF INSECTS 119 



in search of food, selects from a mass of alga? one or 

 two kinds, feeds by flowing round them, and ultimately 

 discards the indigestible residue. Moreover, it is mani- 

 festly affected by changes of temperature, being inert 

 at freezing-point, but becoming more and more active as 

 the water acquires a genial warmth. An allied form, the 

 "slipper-animalcule," is impelled through the water by 

 vibratile hairs (cilia) which beset the surface of its body. 

 It is capable of three adjustments to its surroundings. 

 Normally it moves forward ; but if it encounters an 

 obstacle, its cilia immediately reverse their motion, with 

 the result that the creature moves backward for a short 

 distance. It then stops, revolves on its axis through the 

 fraction of a circle, and once more glides forward. If it 

 again collides, it repeats the manoeuvre of backing and 

 turning until at last its course is clear. 



Such facts as these show us that plants and the lowest 

 animals, although they have no recognisable brains and 

 nerves, are nevertheless well able to maintain their place 

 in the great world of which they form a part. We 

 are also led to conclude that sensitiveness, or irritability, 

 as it is often called, is a fundamental property of proto- 

 plasm — the living, jelly-like substance from which all 

 organisms are built up. Any change in the environment 

 of a plant or an animal, whether it be of temperature, 

 light, or atmospheric pressure, is termed a stimulus, while 

 the corresponding adjustment of the organism is known as 

 its reaction or response. We must not assume, however, 

 that an organism is susceptible to all the innumerable 

 variants of heat, light, moisture and what not which play 

 upon it, or that the same stimulus always evokes the same 

 kind of response. On the contrary, each being, whether 

 plant or animal, is attuned to a particular set of stimuli, 

 to which it responds in certain definite ways. 



