210 



BIOLOGY AND HUMAN LIFE 



gether instead of drifting apart upon being formed by the split- 

 ting of the mother cell. In the animals related to corals and 

 jellyfish (Fig. 44) there is a beginning of this division in that 

 the outer layer of cells shows more sensitiveness, while the inner 

 layer is more active in digesting food (see Fig. 105). x^mong 

 plants one of the earliest divisions found is that between the 

 vegetative cells (those that have to do with the making of food) 



and the reproductive cells 

 (see Fig. 106). Later divi- 

 sions are seen in root and 

 shoot ; and the shoot di- 

 vides into stem and leaf. 

 The leaf divides into pro- 

 tective tissues and veg- 

 etative tissues; the latter 

 again into transportation 

 tissues and photosynthetic 

 tissues. 



In higher animals the 

 division of labor has re- 

 sulted in the development 

 of many kinds of organs 

 —locomotive, protective, 

 food-getting, food-crush- 

 ing, digesting, distributing, 

 storing, waste-removing, 

 and so on. It is interesting 

 and helpful to remember 

 that in every case function precedes structure ; that is, digestion 

 (for example) went on in living things long before there were any 

 digesting organs ; breathing went on long before there were any 

 gills or lungs ; excretion went on before there were any kidneys ; 

 animals moved about before there were any legs or wings or fins.^ 



^This idea is true also if we apply it to the division of labor in society or 

 in the community. Clothes were made long before there were any tailors; food 

 was prepared before there were any cooks; and so on. 



Fig. 106. Volvox 



This organism consists of a hollow sphere made 

 up of a single layer of cells connected bj^ strands 

 of protoplasm. The colony moves about in the 

 water by means of cilia, or vibrating proto- 

 plasmic threads. Each cell contains chlorophyl. 

 Groups of cells (represented by the dark 

 spots) separate from the wall of the hollow 

 sphere and produce reproducing cells 



