26 ANIMAL BIOLOGY 



and energy taken from its environment, constructs more proto- 

 plasm — endows the non-living with its own unique organization. 

 It makes more life-stuff. And, if the available materials are ade- 

 quate, the living substance tends to increase indefinitely, or until 

 the specific limits of the cell or organism are attained. 



5. Reproduction 



So far as is known, living matter arises only by the activities 

 of preexisting living matter. We have seen that this transforma- 

 tion is continually going on in anabolic processes in the animal 

 or plant, and brings about repair and growth of the individual; 

 but it is in reproduction that what may be termed the overgrowth 

 of the individual results in the production of a new one. 



Thus reproduction and growth are phenomena which are in- 

 trinsically the same — both are the result of a proponderance of 

 the constructive phase of metabolism. The single cell, whether a 

 whole organism or a single unit of a complex body, increases in 

 volume up to a certain limit and then divides. In the former case 

 two new individuals replace the parent cell ; in the latter, the com- 

 plex body has been increased to the extent of one cell. In both 

 cases cell division has resulted in cell reproduction. Thus cell 

 division is always reproduction, though it is customary and con- 

 venient to restrict the term reproduction to cell divisions which 

 result in the formation of new individuals — single cells or groups 

 of cells which sooner or later separate from the parent organism. 

 They are the new generation. This is a unique characteristic of 

 living things which provides for the continuation of the race. 

 (Figs. 8, 164.) 



6. Irritability and Adaptation 



The discussion of metabolism has emphasized the close interre- 

 lationship between the living organism and its surroundings, and 

 the dependence of life upon the interplay and interchange between 

 protoplasm and its environment. As a matter of fact the plant or 

 animal retains its individuality — lives — solely by its powers of 

 developing and maintaining exquisite adjustments to its surround- 

 ings. This results from the irritability of living substance: its 

 inherent capacity of reacting to environmental changes by changes 

 in the equilibrium of its matter and energy. The inciting changes, 

 known as stimuli, may be chemical, electrical, thermal, photic, 



