108 DEVELOPMENT AND HEREDITY. 



and density cause currents in the medium. Sur- 

 rounding objects are set in motion, and perhaps the 

 organism itself is moved about. Some of the forces 

 which act upon the organism are constant ; such 

 are gravity and the pressure of the medium. Other 

 forces are regularly periodical, as the light and heat 

 of the sun. Still other forces are intermittent and 

 irregularly periodical, such as the contact and col- 

 lision with surrounding bodies, the impulses that 

 shove and transport the organism, and the contact 

 with particles of nutriment. Some of this last class 

 of forces may act very rarely, others at lesser in- 

 tervals, and still others will act very frequently. 

 Each of the forces which act upon the organism 

 — be it constant, regular, or irregular — will pro- 

 duce its own peculiar mechanical reaction in the 

 mass of living matter. Thus some of the reactions 

 will be repeated at regular intervals, others will be 

 repeated irregularly with greater frequency. In or- 

 der to understand the effects of this repetition, we 

 must recall to mind that peculiar property of living 

 matter which we have called elementary nervousness, 

 which makes possible and necessary the formation 

 of co-ordinations and associations as the result of 

 repetition of the necessitated reactions. As already 

 pointed out, this property seems to be coextensive 

 with organic life, and we must therefore give it 



