58 Resemblances and Differences Among Living Things 



There is of course a limit to the growth of most plants 

 and animals. Among human beings, for example, we recog- 

 nize that the individual reaches a certain size and may re- 

 main for a long time at this point with very slight variations 

 from day to day. Among other living things, such as cer- 

 tain kinds of trees and certain kinds of fish, growth seems to 

 continue as long as life lasts. Whether the growth is rapid 

 or slow, or even if the size is practically stationary, the living 

 being must continue (if it is to remain a living organism) 

 to take in material from the outside and to discharge waste 

 substances to the outside. The chemical changes going on 

 during such a stationary period are essentially all of the same 

 kind as those going on during growth. Moreover, the chemi- 

 cal processes occurring in living things during growth and 

 during maintenance are essentially the same in plants as in 

 animals. 



Different then as the eagle may be from the jellyfish in 

 outward appearance and in the character of its perform- 

 ances, there are certain fundamental similarities. Each has 

 a form characteristic of its species. Each starts life as a very 

 tiny individual — too small to be seen without a microscope. 

 Each grows from this to a form and size characteristic of 

 the species. Each takes in various materials from the sur- 

 roundings and throws out various other substances as part 

 of the fact of growing and keeping alive. 



Movement 



One of the most striking facts about familiar animals 

 is that of movement. Indeed, to most people movement is 

 so closely associated with the idea of animal life that we are 

 apt to assume life wherever we see movement in nature. For 

 the same reason we are apt to overlook life among many 

 plants and stationary or sluggish animals. The capacity to 

 move, however, is present in all living things, plants as well 

 as animals. The movements seem to arise from happenings 

 within the organism, as distinguished from such movements 



