CHAPTER III 



MULTICELLULAR ORGANISMS AND DIVISION 

 OF LABOR 



Nature of the Subject. — Since protoplasm is the 

 only living substance, it follows from what we have 

 learned in the foregoing chapter that living, in a biologi- 

 cal sense, consists of sensation, movement, waste, repair, 

 growth, and the multiplication of individuals. At the 

 same time it is equally true that we recognize certain dif- 

 ferences among living things that lead us to separate 

 them into two great groups, animals and plants. We also 

 distinguish certain peculiarities that enable us to sub- 

 divide the members of these groups into many different 

 species or kinds of living things. We know that for some 

 reason the squirrel is different from the oak tree up which 

 it scampers, and the bee is unlike the clover from which 

 it gathers honey. 



Since these organisms and hundreds of thousands of 

 other species are living, and hence are carrying on the 

 same vital processes, it is well to ask ourselves the ques- 

 tion, on what do these (hfferences rest? The answer is, 

 on structure. All of the different kinds of living things 

 are vital engines, carrying on processes whose sum total 

 we call life. Some are fitted to work in water, others 

 on the surface of the earth or on the ground, while still 

 others spend a portion of their existence in the air. The 

 plant machines are usually stationary, while the great 

 majority of animals move about. Furthermore, many of 

 these machines are comparatively simple, the work they 

 do is rather crude, and we speak of them as low forms of 

 life. On the other hand, the oak tree or the squirrel, for 



24 



