Anabolism and Sjjeciali^ation. 197 



evolved. To repeat, the greater the katabolism the greater 

 must be the anabolism, and in animals the great katabolism 

 implies at the same time a great anabolism and a great 

 instability in the products of the anabolism. Thus the 

 course of evolution in animals might be stated as follows, in 

 terms of metabolism : — 



1. Simple anabolism — growth without differentiation. 



2. Anabolism of more unstable type — growth with dif- 



ferentiation. 



3. Anabolism of such unstable type that katabolism keeps 



pace with it — differentiation without growth. 



4. Preponderant katabolism — senility and death. 



In animals this instability or plasticity would at 

 length by specialisation become so marked as to be in- 

 capable of self-government, or what might be termed 

 organic coherence, just as in chemistry certain compounds 

 may be built up by synthesis until their instability becomes 

 so great that their chemical coherence is broken by the 

 slightest stimulus. 



In the case of plants, where the specialisation would be in 

 the direction of greater stability in the products of anabolism, 

 the course of evolution in like terms would be : 



1. Simple anabolism — growth without differentiation. 



2. Cheaper methods of anabolism entailing less expendi- 



ture and greater stability in its products — growth 

 with differentiation. 



3. Cheaper methods of anabolism — differentiation without 



growth. 



4. Death from complete passage of the protoplasm into 



more stable products of anabolism. 

 The only cells which can be said to retain their vitality 

 for long periods in the life of the higher plants are the 

 meristematic cells of the growing points and of the cambium. 

 The cells which spring from these, and undergo specialisation 

 into conducting, supporting, and storage tissues, and into the 

 special assimilatory organs for carbon, etc., are, in different 

 degrees, constantly undergoing death, and are replaced by 

 others derived from those of the growing points. These 

 specialised cells, even after death, may be used for the 



