CHAPTER II 



THE SENSORI-MOTOR SYSTEM 



We must not attempt to set up any absolute distinctions 

 between plant and animal organisms, or even between one group of 

 animals and another one. Birds, for instance, appear to be quite 

 different in regard to their structure and habits from reptiles, but 

 when one considers the extinct species known to us by their fossils 

 the differences that we see in the living animals are largely 

 obliterated. Typical plants, such as an ash tree or a toad- 

 stool, differ greatly from typical animals such as a fish or a 

 spider, but there is no absolute difference between some of the 

 microscopic alga) (which are plants) and some of the microscopic 

 organisms called infusoria (which are animals). That which we 

 must regard 'as characteristic of plants on the one hand, and 

 animals on the other, are the tendencies that they display. 



Thus the typical plant tends to be a rooted, sedentary organism. 

 As a very general rule its movements are those of growth, and 

 these movements are orientated. The roots tend to grow 

 downwards in the line and towards the direction of the earth's 

 gravitative force, and the stems tend to grow upwards towards 

 the direction from which light mainly comes. Other kinds of 

 movements occur, such as the turning of tendrils round fixed 

 supports, the opening and closing of some flowers, and the 

 motions by which the pitcher plant traps insects, but these are 

 not very common; they are not typical, and when we analyse 

 them we can place them in the same category as the movements 

 called tropisms (those of the root and green stems). 



Animals, on the other hand, tend to be freely mobile. There 

 are many exceptions, of course; thus the zoophytes, corals, sea 

 anemones, etc., are fixed, sedentary organisms, and so also are 

 many kinds of parasites (fish lice and tapeworms, for instances). 

 But the great majority of animals tend to move about; they are 

 locomotory, and their movements exhibit a certain general 

 character which is fairly well described by the term " animal 

 behaviour " in the popular significance of those words. The 

 motions included in such a term are often deliberate and chosen 



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