54 THE QUANTITATIVE METHOD IN BIOLOGY 



A. The arrested frimordia, the growth (development) of 

 which is arrested when they have reached a certain value, with- 

 out any further increase or modification. Examples : The total 

 length of a Spirogyra or of a petal. The number of scales of a 

 hibernaculum, etc. 



B. The transitory primordia, which are transformed at 

 a certain moment into a new (metamorphic) primordium. 

 Examples : In the limb of the corolla of Myosotis palustris 

 three colours appear successively : white, rose, blue. The 

 primordium white is original (with regard to the corolla) and 

 transitory. The primordium rose, being very probably pro- 

 duced by a transformation of a chromogen which is aheady 

 existing during the white period, is metamorphic ; it is also 

 transitory, because it is transformed into the primordium blue. 

 The primordium blue is therefore metamorphic and also arrested, 

 because the blue colour, when it has reached a certain intensity, 

 is not subjected to any further modification. Such is the 

 succession of the colours in the ordinary blue subspecies of 

 Myosotis palustris. In the rose subspecies the blue colour does 

 not appear ; the property rose is therefore arrested. In the 

 white subspecies the property white is arrested.^ 



The arrested properties in their turn may be divided into two 

 groups : 



Aa. The persistent properties, which are preserved till the end 

 of the life of the individual (or part) in which they are observed. 

 Example : The blue colour in Myosotis palustris. 



Ab. The caducous properties, which disappear at a certain 

 moment. Their disappearance is often a direct consequence of 

 the fact that the parts (segments, organs) to which they belong 

 are caducous. Example : The leaves of many plants are 

 clothed with hairs which faU off at a certain moment : all the 

 primordia of the hairs and the primordium hairy itself dis- 

 appear with the hairs. In a similar way all the primordia of 

 the scales of a hibernaculum disappear when the scales fall off 

 in the spring. In the course of the development of almost 

 all (perhaps all) higher animals a number of primordia are 

 caducous,^ owing to the resorption (degeneration, phagocytosis, 

 etc.) of certain organs. 



A primordiimi which disappears gradually may be called 

 deciduous. 



1 With regard to the colour (all other properties being excluded), the white 

 subspecies is infantile compared with the rose, and the rose is infantile com- 

 pared with the blue . (See §45.) 



The succession of the colours in the blue subspecies is an example of segrega- 

 tion of three primordia. (See § 38. ) About the transformation of red into blue, 

 see § 133. 



" A caducous property is, of course, something quite different from a 

 transitory property. 



