PKEFACE TO PART II 



DEVELOPMENT consists of growth and differentiation, accom- 

 panied in the larger organisms by nuclear- and cell-division. 

 The present Part deals with growth. 



The importance of the study of growth cannot be over- 

 estimated, and it is a cause for wonder that the treatment of 

 the subject has been so much neglected by text-books. Indeed, 

 it is a surprising fact that it has not been thoroughly and sys- 

 tematically investigated. For, in last analysis, the maintenance 

 of the human race depends upon that property which protoplasm 

 among all substances alone displays of increasing itself for an 

 indefinite time and to an indefinite amount. And the possibil- 

 ity of increasing the human race beyond limits that are not 

 far off depends upon a better knowledge of the conditions of 

 growth. The reader has only to consider that the world's 

 supply of 2500 million bushels of wheat, 2000 million bushels 

 of maize, 90 million tons of potatoes, and its untold millions 

 of tons of beef, pork, and fish are reproduced each year by 

 growth. The mineral matters of the soil are being washed out 

 into the sea and are largely lost, but the capacity of growth 

 under appropriate conditions is never lost ; it redoubles as the 

 amount of the growing substance is increased. The only thing, 

 then, which limits growth is the limitations in the conditions 

 of growth. What are these conditions ? This is the important 

 question to which attention has been directed in this Part. 



Aside from this practical interest, the study of growth is 

 important as bearing on the question of the dependence of vital 

 activities, and especially development, upon external conditions, 



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