vii DIRECT AND INDIRECT STIMULATION 377 



as it was exercised in the production of various kinds of 

 sounds, it produced them more and more perfectly, and 

 its capabilities were gradually improved. These capabilities 

 therefore are acquired, they were transmitted from one gene- 

 ration to another ; but as they were developed ever greater 

 changes must have taken place in the powers of contraction 

 of the muscles and the efficiency of the nerves of the organ, 

 and above all in the vocal chords. These changes, on account 

 of the great delicacy of the mechanism, were able to produce 

 great effects, although they are not anatomically demonstrable 

 any more than the special structure of the fingers which 

 helps to determine a peculiar handwriting can be exhibited 

 by dissection. Accidental variations of the vocal chords 

 under the action of selection may have promoted the progress 

 of the evolution. 



CONCLUDING EEMARKS. 



In the second section I gave reasons for the conclusion 

 that the external stimuli which act directly upon the pro- 

 toplasm are the primary causes of the manifold variety among 

 organic forms. On the other hand, as I there explained, I 

 regard the influences of use, of functional activity, which have 

 been discussed in the last and preceding* sections, as the 

 indirect or secondary causes of growth and of the modifica- 

 tion of structure. As the direct influence of stimulation, 

 which I have previously named "impression," promotes 

 growth, so also does exercise, which modifies the protoplasm 

 and, by producing an increased inflow of the nutritive fluids, 

 adds to its bulk. 



But in many cases the effects of direct and indirect stimu- 

 lation cannot be separated. When a stimulus acts constantly 

 in a certain way upon the protoplasm, the latter is exercised 

 in a corresponding manner, and is thereby modified. In fact, 

 when we speak of use as the cause of growth or of the modi- 



