154 INTRODUCTION TO SCIENCE 



(a) In the first place, while we do not know of 

 any transitions between the not-living and the 

 living, we have a long inclined plane or a long 

 series of steps connecting the very simple reactions 

 of unicellular creatures with the intelligent be- 

 haviour of dogs and horses, and even with the 

 rational conduct of man. This inclined plane 

 or this staircase is very impressive, and must 

 have a bearing on the general problem. There 

 is a fascination in what may be called the begin- 

 nings of behaviour, illustrated by some of the 

 Infusorians. Their daily life seems as if it could 

 be summed up in a sentence. They have only 

 one answer to every question. To all sorts of 

 stimuli they respond in the same way by back- 

 ing off, turning slightly on one side, and then 

 going ahead again. They remind us of a steamer 

 in a river which knocks against a snag, reverses 

 engines, alters its direction a little, and then 

 steams ahead. This is surely the simplest kind 

 of behaviour, where there is only one reaction. 



Slightly higher in the scale, but still very simple, 

 is the behaviour of some Protozoa which have a 

 number of reactions or responses to stimuli, and 

 seem to try one after the other until, it may be, 

 one succeeds. We do not know how much lies 

 concealed in that process of " seeming to try." We 

 know that it is different from the experimenting 

 of a scientist who tries various ways of solving 



