400 THE WONDER OF LIFE 



gall-insect which has emerged from its egg-envelope de- 

 posited within the tissue of the leaf or stem. In these cases 

 we have very striking instances of specific secretions induc- 

 ing specific kinds of growth. These are extraneous 

 secretions introduced into the plant by an animal, but 

 we have also evidence of intrinsic secretions within the 

 plant which help to regulate growth. Thus it is said that 

 in the growth of the roots of some plants, specific chemical 

 substances are formed which inhibit further growth. In 

 short, facts are accumulating which show that particular 

 parts of an organism have their growth regulated by specific 

 internal secretions. 



In his Principles of Biology Herbert Spencer devoted 

 much attention to the conditions of growth. He sought 

 to show that growth varies — other things equal — (1 ) directly 

 as nutrition, (2) directly as the surplus of nutritive income 

 over expenditure, (3) directly as the rate at which this 

 surplus increases or decreases, (4) directly (in organisms 

 of large expenditure) as the initial bulk, and (5) directly 

 as the degree of organization. This kind of analysis is 

 valuable, but what is more needed at present is an 

 extensive series of measurements of growth under diverse 

 conditions and in different kinds of organisms. 



It is interesting to inquire into the periods and rates 

 of growth in different organisms. After an egg-cell has 

 been fertihzed it divides and re-divides, but for a time, 

 though there is increase in the number of cells, there is 

 no increase in size. We see development, but no growth. 

 Soon, however, development and growth proceed hand in 

 hand, both very rapidly. Later on, when development is 

 proceeding slowly — ^all the chief steps having been taken — 

 growth may stiU continue very vigorously. Thus in the 



