18 FOUNDATIONS OF BIOLOGY 



taining exquisite adjustments to its surroundings. 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, or mechanical, but the nature of the response 

 is determined rather by the fundamental character of the 

 protoplasmic system itself than by the nature of the stimulus. 

 Muscle protoplasm contracts however it is stimulated. The 

 reaction of living matter by virtue of its intrinsic irritability 

 implies not only response to a stimulus but also conduction 

 so that the protoplasmic system as a whole is directly or 

 indirectly influenced. It responds as a coordinated unit - 

 an individual. It adapts itself structurally and functionally 

 to the exigencies of its existence. . This power of adaptation, 

 as exhibited in active adjustment between internal and 

 external relations, overshadows every manifestation of life 

 and contributes, more than any other factor, to the " enor- 

 mous gap that separates even the lowest forms of life from 

 the inorganic world." 



6. Organization 



Finally, adaptation implies that living things are not 

 homogeneous, but exhibit reciprocal structural and physio- 

 logical organization. Accordingly animals and plants are 

 referred to as organisms. Indeed a major part of the present 

 volume is devoted to the organization of organisms. 



The characteristics which we have described chemical 

 composition, metabolism including waste and repair, growth 

 by intussusception, reproduction, adaptation, and specific 

 organization individually and collectively are diagnostic 

 of living matter. It is possible, to be sure, to take exception 

 to one or another; e.g., to say that growth by intussusception 



