52 THE QUANTITATIVE METHOD IN BIOLOGY 



series of reactions) which results in the production of an adult 

 primordium, two sets of causes may be discerned ( §§ lo and i8) ". 

 on the one hand, internal or specific causes, which depend on the 

 chemical composition of the hving mixture ; on the other hand, 

 external causes, which depend on the environment. The in- 

 ternal causes may be looked upon as being the same in all the 

 specimens of a given species, but the external causes are vari- 

 able from one individual to another. According to this, each 

 adult primordium is a priori variable in a given species, because 

 it is, in each individual, in equilibrium with a particular com- 

 bination of external forces. This equiUbrium is called indi- 

 vidual adaptation or accommodation. Since all the properties 

 of a given individual are accommodated in this way, we may 

 say that the individual, considered as a whole, is accommodated 

 to (in equilibrium with) its conditions of Ufe. An individual 

 is accommodated to its environment, not in order to obtain 

 certain physiological advantages, but because it cannot be 

 otherwise. 



§ 45._EXAMPLES OF INDIVIDUAL ADAPTATION. 



— Several examples of individual adaptations {Polygonum 

 amphibium, Primula sinensis, etc.) are mentioned in §§ 12 and 

 14-17. I call plasticity the variation produced by individual 

 adaptation. 



In the following example one single primordium is taken into 

 account. Suppose that an adult petal a of a given species has 

 a length of 12 mm. : in the course of its development the length 

 has been successively i, 2 ... 9, 10, 11, 12 mm. When the length 

 12 mm. was reached the primordium length was in equilibrium 

 with the environment. Suppose now a second adult petal b of 

 the same species the length of which is 10 mm. The petal b 

 has been subjected to different influences ^ during its develop- 

 ment. It passed also through the series of values i, 2 ... 9, 

 10 mm., but it was in equilibrium with its environment when 

 its length had reached 10 mm. 



Both petals have been accommodated to their respective 

 conditions of existence : the difference between them may be 

 expressed by measurement. 



REMARK : Although the shorter petal b is adult, just as 

 the longer one a, we may say that b is in an infantile [juvenile, 

 pcBdogenetic) state with regard to the primordium length 

 because its growth has been stopped in a younger state. 



§ 46.— CLASSIFICATION OF THE PRIMORDIA WITH 

 REGARD TO THEIR DEVELOPMENT.— In the course of 

 the development of a living individual each of its primordia 



1 Temperature, light, food, water, etc. (See § 119.) 



