14 INTRODUCTION 



9. The Difference between Plants and Animals in Respect 

 to Stimulus. Living animal cells possess the property of 

 irritability or excitability, that is, some change in their 

 composition results from the action of stimulus. Vegeta- 

 ble cells also possess this property in some degree. But it 

 is found that, as in the processes of development more and 

 more complex forms of plant life appear, the plant does 

 not develop special organs for the transmission of stimulus. 

 In the animal kingdom, on the other hand, a striking dif- 

 ference appears. In one of the lowest known representa- 

 tives of animal life the amoeba (Fig. 3, p. 16), which 

 is a mere microscopic lump of naked protoplasm each 

 minute particle of the protoplasm appears to respond to 

 a stimulus and to transmit it to the adjacent particles, 

 there being no distinction of parts or functions in the 

 single cell. But in the next higher division of ani- 

 mals, the corals, sea anemones, etc., the rudiments of a 

 nervous system are visible, and some division of sense 

 organs appears. It is probable that nervous impressions 

 are received first in but a single form, while a gradual 

 and uninterrupted development of the senses follows as 

 we rise in the scale. That is, one of the lower animals 

 may be said to have but one sense, touch, or a general sen- 

 sibility, it receives but one kind of sense impression 

 from influences which higher animals recognize as diverse, 

 while higher animals may distinguish two or more kinds 

 of impression, and so on. It should be noticed that the 

 common division of senses into touch, taste, smell, sight, 

 and hearing is somewhat arbitrary, even man not being 

 always able to discriminate, for instance, between taste 

 and smell, while certain sensations are recognized, such 

 as perception of temperature and of pain, which do not 

 strictly belong to any of the " five senses " so called. 



