112 THE RIDDLE OP THE UNIVERSE. 



complex functions of the sense-organs and the brain 

 in the higher animals and man. The activity of the 

 psychoplasm, which we call the " soul," is always 

 connected with metabolism. 



All living organisms, without exception, are sensi- 

 tive ; they are influenced by the condition of their 

 environment, and react thereon by certain modifica- 

 tions in their own structure. Light and heat, gravity 

 and electricity, mechanical processes and chemical 

 action in the environment, act as stimuli on the 

 sensitive psychoplasm, and effect changes in its 

 molecular composition. We may distinguish the 

 following five chief stages of this sensibility : — 



I. — At the lowest stage of organisation the ivhole 

 psychoplasm, as such, is sensitive, and reacts on the 

 stimuli from without ; that is the case with the lowest 

 protists, with many plants, and with some of the most 

 rudimentary animals. 



II. — At the second stage very simple and undis- 

 criminating sense-organs begin to appear on the 

 surface of the organism, in the form of protoplasmic 

 filaments and pigment spots, the forerunners of the 

 nerves of touch and the eyes ; these are found in 

 some of the higher protists, and in many of the lower 

 animals and plants. 



III. — At the third stage specific organs of sense, 

 each with a peculiar adaptation, have arisen by 

 differentiation out of these rudimentary processes : 

 there are the chemical instruments of smell and 

 taste, and the physical organs of touch, temperature, 

 hearing, and sight. The " specific energy" of these 

 sense-organs is not an original inherent property, but 

 has been gained by functional adaptation and pro- 

 gressive heredity. 



