PHYSIOLOGY 163 



"modifications" with fundamentally different peculiarities. In con- 

 sidering living organisms, it is the irritability or living modification 

 of the protoplasmic substance which must occupy the attention. The 

 object, therefore, of Physiology consists principally in discovering 

 the attributes and characteristics incident to the modifications of 

 living protoplasm. 



These attributes and characteristics are so distinctive as to 

 separate by a wide gap living bodies from all other matter. It is, 

 in fact, impossible to form any conception of the manner in which 

 living bodies have arisen on this once molten planet from lifeless 

 matter. Acceptance of the theory of evolution authorises, it is true, 

 the transfer of the inception of life on the earth to geological periods 

 separated by millions of years from the present time ; but the 

 initiative character of such dawning life remains no less incom- 

 prehensible. From a consideration, however, of the attributes of the 

 living substance, it can with safety be said that the external con- 

 ditions of life could not at that time have been so very different from 

 those now existing on the earth ; for it is a characteristic quality 

 of living matter that its vital activity, even its very existence, 

 is circumscribed and limited by external, cosmic influences. The 

 vitality of vegetable protoplasm can only be preserved within a 

 definite range of temperature, within about sixty degrees Celsius, 

 while its full vital activity is restricted to still narrower limits. Too 

 intense light or an insufficiency of water destroys its life ; while the 

 most minute quantities of certain poisons suffice to shatter instantly 

 and irrevocably that mysterious structure, in which, under favourable 

 conditions, lies concealed the capacity to vivify the whole world. 



Although living plants are themselves responsible for the manner 

 in which their vital phenomena manifest themselves, they stand, 

 nevertheless, in the closest reciprocal relations with their environment, 

 upon the condition of which they are altogether dependent. From 

 the outer world they obtain not only their nourishment, but receive 

 also from it, particularly from the vibrations of light and heat, the 

 energy that they again expend in the manifold processes of their vital 

 phenomena. It is to the operation of these external influences that 

 the stimuli are due which constantly call forth in vegetable protoplasm 

 the manifestation of vital phenomena. These external influences, 

 however, are only serviceable to the processes of life when they 

 operate within definite limits of intensity. The lowest limits of 

 intensity for the effective operation of an external influence is 

 designated the minimum, the highest the maximum, while that 

 degree of intensity at which it is most operative in calling forth the 

 most active manifestation of a definite vital phenomenon is termed the 

 OPTIMUM (cf. also p. 234). For the different vital processes of the same 

 plant, and also for those of distinct plants, these so-called cardinal 

 POINTS are generally different. Thus, some plants flourish best when 



