86 ELEMENTS OF BIOLOGY. 



as a vehicle. Every tissue takes from the common store- 

 house of the blood just those materials which it requires, 

 and builds these up into matter similar to itself. The 

 muscles take from the blood the substances necessary to 

 form muscle, the bones take the materials required for the 

 production of osseous tissue, and so on. Every tissue, 

 therefore, possesses the power of replacing the particles 

 destroyed by its functional activity, by manufacturing, so to 

 speak, particles equal in number and similar in character to 

 those which have died. Hence, a tissue may remain for an 

 indefinite period apparently unchanged, though constantly 

 active and constantly suffering loss of substance. Hence, 

 also, as every tissue has the power of thus maintaining its 

 integrity by the assimilation of new matter, the entire or- 

 ganism may remain unaltered in appearance throughout a 

 long period of active life, though actually the seat of inces- 

 sant loss and equally incessant repair. And, in those beings 

 in which the body is composed of a uniform substance not 

 exhibiting any differentiation into distinct organs, the as- 

 similation of the individual particles of the body becomes 

 undistinguishably merged in assimilation by the organism as 

 a whole. ^ 



By means of the power of assimilation, as above described, 

 every organism possesses the power of maintaining a 

 certain average condition during a longer or shorter period 

 of active life, its losses being exactly balanced by its gains. 

 There is, however, a fundamental difference between all 

 animals and the majority of plants as to the powers which 

 they possess of preparing nutrient matter to be subsequently 

 assimilated. All animals, without a known exception, re- 

 quire- to be supplied with ready-made organic compounds 

 for their food. The food need not necessarily contain the 

 exacf organic compounds which the animal requires to build 

 up its tissues. Indeed, in the great majority of cases, if 

 not in all, the organic compounds of the food have to 

 undergo certain changes before they can be actually em- 



