PROTOZOA 291 



wood-work, a third the locks, another the painting, and 

 so on. Then each section knows its own business, and 

 can pursue it with effect. 



The whole organisation of the higher animals rests 

 on the principle of the division of labour. If each cell 

 had to discharge all the vital functions, they would 

 restrict each other. The secretory product only dis- 

 charges one function, and . so is not distracted by other 

 duties in doing so. 



The lower we descend in the animal scale, the less 

 division of labour do we find. Thus, in the animals 

 with which we concluded the preceding chapter, the 

 polyps, we find only two kinds of cells — as a rule, at 

 least — which clothe the inner and outer surfaces of the 

 sac-shaped body. The internal layer of cells accom- 

 plishes digestion ; the external keeps the animal in 

 touch with the outer world. Both of them are equally 

 engaged in movement. In the further course of animal 

 evolution the external stratum of cells was differentiated 

 into skin and nerves ; the internal divided into the 

 alimentary canal, with its dependent glands, and the 

 muscles and bones. 



In harmony with the theory of descent we must 

 assume that in certain ancestors of the polyps there 

 was as yet no division of labour ; that each cell had 

 to discharge every function. And these mulberry- 

 shaped animals must in turn have had ancestors that 

 consisted of a single cell. 



The embryonic development of every animal establishes 

 some such series of ancestors. Every animal begins 

 life as a single cell, the ovum. To this succeeds a 



