THE CYCLE OF LIFE 401 



pre-natal life of man, great strides in development are 

 taken in the first three months, along with very rapid 

 growth. Thereafter, when the developmental steps are 

 much less striking, the growth is for a time very rapid. 

 From the third to the fourth ante-natal month, the increase 

 is 600 per ceBtt. After this it drops quickly and is barely 

 25 per cent, in the last month before birth. 



In some organisms the growing period is very sharply 

 punctuated ; thus in insects with complete metamorphosis 

 all the gromng is done in the larval period. , After 

 the fully-formed winged insect emerges from the pupa- 

 stage, there is no increase in size. This holds good in all 

 butterflies and moths ; ants, bees, and wasps ; beetles; 

 and two-winged flies. In many cases the adult does not 

 feed at all, and there is the sharpest contrast between 

 the larva which feeds, grows, stores, and moults, 

 and the adult or imago which does not grow or moult, 

 but is especially concerned with the continuance of the race. 



In other cases growth appears to have no limit but 

 the length of the Hfe-tether. As long as the organism hves 

 and feeds it may go on growing. Thus we may distinguish 

 the indefinite or indeternainate growth of fishes and reptiles 

 from the definite or determinate growth of birds and 

 mammals. A sacred crocodile may continue slowly growing 

 year after year, and, it is said, decade after decade. It is 

 not uncommon to get huge haddocks as large as good-sized 

 cod-fish, but there is very little variation in the size of a 

 sparrow or of a squirrel. In other words, some organisms 

 show a very definite Umit of growth — ^the physiological 

 optimum — ^while others do not. 



An interesting feature about growth is the occurrence 

 of minor periodicities. Partly no doubt because of its 



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