GROWTH AND REPRODUCTION. 2 oj 



lation of nutritive material, correspond to a predominance of 

 protoplasmic processes, which are constructive or anabolic. The 

 growing disproportion between mass and surface must, however, imply 

 a relative decrease of anabolism. Yet the life, or general metabolism, 

 continues, and this entails a gradually increasing preponderance of 

 destructive processes, or katabolism. As long as growth continues, 

 the algebraic sum of the protoplasmic processes must of course be plus 

 on the side of anabolism, and growth may be now more precisely 

 defined as the outcome of the preponderance of an anabolic tendency, 

 rhythm, or bias. The limit of growth, when waste has overtaken and 

 is beginning to exceed the income or repair, corresponds in the same 

 way to the maximum of katabolic preponderance consistent with life. 

 The limit of growth is the end of the race between anabolism and 

 katabolism, the latter being the winner. Thus cell-division occurs 

 especially at night, when nutrition is at a stand-still, and when there 

 is therefore a relative katabolic preponderance ; and so explorers have 

 shown us that many marine algae reproduce during the darkness of 

 the Arctic winter. 



What is true for the cell, is true for cell-aggregates. Organisms in 

 their entirety have very definite limits of growth. Increase beyond 

 that takes place at a risk, hence giant variations are peculiarly unstable 

 and short-lived. Or again, just as the single cell has found, probably 

 somewhat pathologically, a surface- gaining expedient in the emission 

 of mobile processes, so many organs, notably leaves, have struck a 

 balance between mass and surface by becoming split up into lobes and 

 more or less discontinuous expansions. 



Spencer has laid great stress on the importance of the physiological 

 capital with which the organism begins ; this represents, in active 

 animals at least, the start which their anabolism gets at the outset. 

 Other things equal, growth varies — (a) directly as nutrition ; (b) 

 directly as the surplus of nutrition over expenditure ; (r) directly 

 as the rate at which this surplus increases or decreases; (d) directly 

 (in organisms of large expenditure) as the initial bulk ; and (e) directly 

 as the degree of organization, — the whole series of variables being 

 finally in close relation to the doctrines of the persistence of matter 

 and conservation of energy. Some apparent exceptions are readily 

 explained. Thus, many plants seem to grow indefinitely, but they 

 expend very little energy, and have often enormous surface-area in 

 proportion to mass. The crocodile goes on slowly growing, though 

 at a gradually diminishing rate, but it again expends relatively little 

 energy in proportion to its high nutrition. Birds which expend most 

 energy, have their size most sharply defined. 



