38 PHYSICAL EXPRESSION. 



structural points are of more importance than 

 points of colour. The colour is the result of the 

 molecular condition of the surface ; the general 

 structure is a more widely spread physical con- 

 dition. 



Sound is one of the commonest modes of expression 

 among animals. The sounds produced are usually 

 indirect expressions. 



It is very interesting to study changes in the 

 functions of an organism or in part of a living- 

 being, and such studies are very important in any 

 investigation of the processes of development. We 

 have, then, to look to the signs or expressions of 

 change of function in an organism, and the most 

 satisfactory kind of evidence is that afforded by 

 " direct expression," or some change in the outcome 

 of the action of the subject which can be directly 

 observed as an objective sign. A change of function 

 in parts of the body of a plant or animal is not 

 an uncommon occurrence, and may serve as a means 

 of expression. In the life-history and development 

 of many species a change of function is uniformly 

 met with. A few examples may be mentioned, 

 taking together the change of function and its 

 expression in objective signs. 



The .case of change in function in the cells of 

 the pulvinus of Oxalis has been already given in 

 chap. ii. p. 26. 



The body is not fully developed in childhood, and 

 the hair-bulbs in the skin of the face take on active 

 growth about the time of the development of full 

 manhood. In a woman the growth of hair-bulbs in 



