lines of differentiation. One line departs from a more primitive life by special- 

 izing in vegetative activities; the other specializes in using up food (see 

 frontispiece.) 



Complex systems of organs make possible a greater variety of life outside 

 the water. We have seen how ferns and seed plants managed to free themselves 

 from dependence upon constant wetness (see page 386). Each land animal in 

 effect carries about in its body a section of the primitive ocean, so that it is 

 able to tolerate a great deal of variation in external moisture. Birds and 

 mammals maintain a fairly uniform temperature and a fairly uniform rate of 

 metabolism on the inside, in spite of the great changes in outside temperature. 

 In these respects the "higher" animals are free from the constant changes in 

 temperature and moisture, which constantly suspend or stop metabolism in 

 simpler organisms. 



Food-getting, protecting, body-building, and other processes are ap- 

 parently carried on more efficiently in organisms having specialized organs 

 and tissues. It is true that in the common plants and animals a considerable 

 part of the body consists of nonliving materials, such as wood and bark or bone 

 and shell. Nevertheless such an organism can grow a much greater total 

 of living matter from a single cell in a season, or in the course of years, than 

 can a simple organism that is nearly all protoplasm — like an ameba, for 

 example. 



"Division of labor", or specialization of functions, operates in an organism 

 about as it does in human society. Through becoming specialized, each unit 

 carries out its particular processes more efficiently, although it neglects others. 

 It can produce a surplus of its specialized product or services. It can continue 

 to live, however, only in co-operation with other specialized units. The ex- 

 changes and co-ordinations of the many different organs use up materials and 

 energies. This is like the fact that modern industrial and commercial life uses 

 up more work and materials than older ways in hundreds of tasks that are 

 not directly "productive" — transporting, communicating, recording, account- 

 ing, managing, and so on. But these additional needs are more than made up 

 for by the increased effectiveness of the total. 



Thus a blood system consists only in part of living protoplasm; a bone 

 system carries on very little "growth" after it has reached full size. Yet the 

 blood makes possible a much higher degree of effective brain and muscle and 

 gland work in all parts of the body than the various cells could carry on as 

 independent units. The bones make possible the building up of masses of 

 protoplasm that could not otherwise hold together. The greater the degree 

 of specialization, the greater the amount and also the intensity of living. 



Vegetative and Reproductive A one-celled organism absorbs and assimi- 

 lates food, and grows: that is vegetation. Past a certain point the cell does 

 not grow further, but divides into two cells. While we might call this act 



418 



