THE STRUCTURE OF LIVING THINGS. 19 



bony skeletons ; sometimes for protection as in oyster- and snail 

 shells ; sometimes apparently from sheer inability on the part of 

 the protoplasm to get rid of it. Thns we see that youth is lit- 

 erally the period of life and vigor, and age the period of com- 

 parative lifelessness. 



Summary. The bodies of higher animals and plants are 

 subdivided into various parts {organs) having different structure 

 and functions. These mav be resolved into one or more tissues^ 

 each of which consists of a mass of similar cells (or their deriva- 

 tives) having a similar function. The cells are small masses of 

 living matter, or protoplasm, which dej)Osit more or less lifeless 

 matter either around (outside) them or within their substance. 

 In the former case the protoplasm may continue to live, or it 

 may die and be absorbed. In the latter case it may likewise live 

 on for a time, or may die, either disappearing altogether or leav- 

 ing behind a residue of lifeless matter. 



The Organism as a Whole. Up to this point we have con- 

 sidered living organisms from an anatomical and analytical stand- 

 point, and have observed their natural subdivisions into organs, 

 tissues, and cells. We have now only to remark that these parts 

 are mutually interdependent, and that the organism as a whole 

 is greater than any of its parts. Precisely as a chronometer is 

 superior to an aggregate of wheels and springs, so a living organ- 

 ism is superior in the solidarity of its parts to a mere aggregate of 

 organs, tissues, and cells. We shall soon see that in the living 

 body these have had a common ancestry and still stand in the 

 closest relationship both in respect to structural continuity and 

 community of interest. 



