IRRITABILITY 253 



sensitive to all the forces and influences which we combine 

 without analysis under the name environment, and that the 

 cooperation of these influences induces, as reactions in the 

 living plants, the qualities which we see and call character- 

 istic of the species, order, or class — reactions which are 

 really characteristic only of the living protoplasm. Proto- 

 plasm is not all equally sensitive to any one influence or to 

 the whole complex of influences which constitute its living 

 and lifeless environment. These different degrees of sensi- 

 tiveness, coupled with different powers of response, are what 

 bring about, in the same environment, the forms, sizes* 

 colors, etc., which characterize individuals and species. 



The various forces operating upon the living organism set 

 up and maintain in it physical and chemical conditions 

 which can be changed only by the introduction of a new 

 force or by a change in the relative proportions of the old 

 forces. The living differs from the dead organism and from 

 aU lifeless compounds and structures in the supreme deli- 

 cacy of its structure, due to the arrangement, complexity, 

 and instability of the compounds composing and contained 

 within it (see pp. 183-6). The sensitiveness of living or- 

 ganisms is different in degree and not in kind from that 

 of lifeless things; the sensitiveness of both is a matter of 

 physics and chemistry. 



