GENERAL PHENOMENA OF LIFE 33 



the power of adapting itself to the new conditions, provided, of course, 

 that the change to which it is subjected, is not extreme. Hence, our 

 conception of life is limited to such phenomena as metabolism, growth 

 and evolution, reproduction, irritability and contractility, inclusive 

 of motion. 



General Conditions of Life. The reason for the great diversity in 

 the form of living matter must be sought in the conditions under which 

 it is made to exist. Any change in the latter varies its metabolism, 

 shape and energetics, but naturally, it would lead us altogether too 

 far to study the different aspects of life in detail. In general, however, 

 it may be stated that living matter presents certain internal as well as 

 external characteristics. Among the former might be mentioned its 

 structure, composition and physical properties, the study of which 

 would necessitate an analysis of the cell and its component elements. 

 Among the latter are to be noted the different conditions under which 

 the cell is made to live, inclusive of the character of the medium, the 

 temperature, the atmospheric pressure, osmotic pressure, moisture, 

 and store of nutritive material. 



As long as these conditions remain the same, life is said to be spon- 

 taneous. This term, however, is not a very good one, because life 

 is never actually unconditioned. Thus, an organism leading appa- 

 rently a perfectly spontaneous existence, is constantly under the in- 

 fluence of internal and external conditions. Its spontaneity, therefore, 

 is only apparent, owing to the fact that the influences acting upon it 

 are normal in their character and remain constant in their intensity. 

 On the other hand, if the latter suddenly assume a different quality or 

 become augmented by new conditions, the spontaneity immediately 

 gives way to phenomena of stimulation. Hence a stimulation must 

 result whenever the conditions of life are suddenly and markedly al- 

 tered. In view of the fact that the latter are very numerous and 

 relatively inconstant, the possibility of stimulation is always present, 

 provided the protoplasm retains its receptive power. 



Upon this basis, a stimulus may be defined as any extraordinary 

 change in the conditions to which an organism may be subjected. 

 While the number of stimuli is practically unlimited, it is possible to 

 arrange them qualitatively in the following manner: 



* 



(a) Mechanical stimuli, inclusive of such influences as touch, pressure, stroking, 

 pulling, the effects of gravitation, cohesion and adhesion, etc. 



(6) Chemical stimuli, produced by various normal and abnormal substances. 

 Among the former may be included the nutritive substances ordinarily required by 

 living matter, and among the latter, practically any chemical agent with which it is 

 accidentally brought into contact. 



(c) Osmotic stimuli, consisting in changes in the osmotic pressure of the sur- 

 rounding medium. As these alterations are commonly associated with chemical 

 reactions, they are frequently included among the former. 



(d) Thermal stimuli, produced by variations in the temperature of the medium. 



(e) Photic or radiating stimuli, caused by changes in the intensity and quality 

 of the light. Under this heading may also be placed the peculiar rays discovered 



3 



