MEASUREMENT OF VARIABLE PROPERTIES 167 



seems to have the same significance as in an inorganic variation 

 curve. The length of the adult spike of the rye being taken 

 as example, the external causes (temperature, water, food, etc.) 

 which bring about variation by their combinations are in 

 reality the causes of the development of the spike.^ Without 

 heat, water, food, etc., no spike would come into existence, 

 and no mean value would exist. 



There is, however, between both cases (inorganic and bio- 

 logical) a fundamental difference. 



§ 119.— BIOLOGICAL AND INORGANIC VARIATION 

 CURVES (continued): — Among the external causes (factors) 

 which have (or may have) an influence upon the development 

 of the living beings, the following may be mentioned : — 



(i) Temperature. 



(2) Light (source of energy and stimulant). 



(3) Water. 



Food, which consists of a number of substances ; for 

 instance, (4)- (15) : 



(4) Carbon (tension of CO^). 



(5) Nitrogen. 



(6) Phosphorus. 



(7) Sulphur. 



(8) Chlorine - (Br., L). 



(9) Potassium. 



(10) Sodium. 



(11) Calcium. 



(12) Magnesium. 



(13) Iron. 



(14) Oxygen (food and respiratory comburant ; tension of 

 oxygen in the atmosphere). 



(15) Silica. 



(16) Electricity. 



(17) Radio-activity. 



(18) So-called catalysts (manganese, lead, uranium, etc.). 



(19) Other living beings (parasitism, symbiosis properly so 

 called, etc.). 



(20) The space available for the development of the indi- 

 vidual. 



Each of the mentioned causes (the list is rather incomplete) 

 has its proper influence upon each living specimen considered 

 as a whole and upon each of its primordia. These causes are 

 the CONDITIONS OF EXISTENCE. 



In the course of the development of a living specimen the 



1 without the external factors the (internal) specific causes would not be 

 operative. 



