ORGANIC CHANGES 71 



in the term metabolism, the two aspects of which constructive 

 and disruptive are called anabolism and katabolism. 



2. Cyclic Changes. An equally familiar fact is that organisms 

 pass through a series of changes. The fertilised egg undergoes 

 cleavage, the resulting cells grow and differentiate, an embryo 

 is formed, and gradually often by circuitous paths a minia- 

 ture form of the adult creature is attained. Out of apparent 

 simplicity an obvious complexity results. Growth still con- 

 tinues, often punctuated by resting periods, often rhythmic and 



FIG. 17. Diagram illustrative of variation and modification. 



S, the soma or bod}' ; G. the germinal material ; E, an environmental change. 



A, an environmental change acting on the body directly evokes a modification (M). 



B, an environmental change, without modifying the body directly, acts as a stimulus on 

 the germ-plasm, and is followed by a variation (V). 



C, a variation (V) arises from some germinal change which cannot be causally connected 

 with any particular environmental change. 



expressible in complex curves, often interrupted by peculiar 

 crises. Quickly or slowly the organism passes from youth 

 through adolescence to maturity, to its limit of growth and its 

 reproductive maturity. Quickly or slowly thereafter it sinks on 

 a down-grade towards death. As the old naturalists said, from 

 one period of vita minima the creature rises to a period of vita 

 maxima, and sinks back again into a vita minima which 



