SECT, ii PHYSIOLOGY 175 



vital processes. It is, then, this peculiar quality of irritability 

 that distinguishes living protoplasm from other bodies, and con- 

 stitutes the fundamental distinction between living and dead proto- 

 plasm. Such a view is, however, not contrary to accepted ideas ; 

 simple chemical bodies, indeed even chemical elements, such as 

 sulphur, phosphorus, etc., exist in different " modifications " with 

 fundamentally different peculiarities. In considering living organ- 

 isms, it is the irritable 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 character 

 and condition of origin of such dawning life remain no less incompre- 

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

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

 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 tempera- 

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

 Too intense light or too little warmth destroys its life ; while the 

 most minute quantities of certain poisons suffice to shatter instantly 

 and irrevocably that mysterious structure, in which, under favour- 

 able conditions, lies concealed the capacity to populate 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 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, how- 

 ever, are only serviceable to the processes of life when they operate 

 within definite limits of intensity. The lowest limit of intensity for 

 the effective operation of an external influence is designated the 

 MINIMUM, the highest the MAXIMUM, while that degree of intensity 



