EXPRESSION BY CHANGE IN FUNCTION. 39 



the face is usually delayed till old age. The func- 

 tions of the stomach, and its power of digestion, 

 vary at different periods of life, and under different 

 circumstances : the stimulus of food causes an acid 

 secretion to be poured out ; when empty, the secre- 

 tion of the organ is alkaline. In the foetus the lungs 

 are not used as respiratory organs, and at birth the 

 salivary glands are not active. Ants, when first 

 born, possess wings which they soon learn to use, 

 but after a short time they lose their wings and 

 become solely terrestrial animals for the rest of 

 their days. A bar of iron may be magnetized, and 

 its properties are thus changed. In vegetable 

 growth it is very common to find examples of 

 epinasty and hyponasty* alternating, the cells 

 growing first on one side, then on the other. 



In man certain changes in the larynx in the 

 course of growth cause an alteration in the voice. 

 The action of glands varies from time to time ; e.g. 

 secretion of mucous membranes, etc. If a man is 

 kept long in the dark his retina becomes over- 

 sensitive to light. Definite and permanent changes 

 in the structure and functions of a part are more 

 clearly seen in a species than in an individual ; this 

 is owing to the greater length of life of a species as 

 compared with an individual. 



Movements are often exceedingly expressive. 

 When observing a man, for the purpose of forming 

 a judgment as to his mental condition, and deter- 

 mining what emotion is most active within him, we 

 take note of his gestures or movements. An actor 



* Sachs, " Text-book of Botany," 1875. p. 767. 



