44 THE QUANTITATIVE METHOD IN BIOLOGY 



variation in the conditions of existence results in a series of 

 transitions between the parental species (with regard to the 

 observable properties) (§ 35, p. 38). 



In the preceding examples ^ segregation (produced by plasticity) 

 brings about differences between specimens. It may also happen 

 that segregation results in a difierence between two parts of 

 one specimen. Thi? occurs in a certain sense in the case of 

 Philodendron, in which the specimens P and p are, in fact, parts 

 of one plant. In a similar way, when the crown of a copper 

 beech becomes large enough, the leaves which are developed 

 in the shady central part of the crown are green, while the 

 peripheral leaves of the same crown, being exposed to sun- 

 shine, are red. 



Some sorts of apples are green or greenish when ripe ; other 

 sorts are red. It often happens that in a fruit of a red sort the 

 side which is exposed to light becomes red, while the opposite 

 side is green. Both parts of the apple differ in the properties 

 red and green, in the same way as two plants of the F2 genera- 

 tion after a (supposed) cross between a seed-fixed red sort and 

 a seed-fixed green sort. Similar differences between two parts 

 of one specimen are observed in certain hybrids : among the 

 hybrids produced by a cross between the ordinary Antirrhinum 

 majus with personate flowers and the subspecies with peloric 

 flowers certain specimens bear simultaneously personate and 

 peloric flowers. 



Sometimes plasticity causes segregation in more than two 

 groups of specimens. In Polygonum amphihium we observe : 

 (i) a terrestrial form (stem erect, etc.) ; (2) an aquatic form 

 (stem flexible, etc.) ; (3) a xerophytic form (stem creeping, 

 etc.) (See § 12.) 



Decomposition of a Compound Property into Primordia by 

 Plasticity. — We have seen in § 36 that a compound property 

 may be dissociated into simple properties by hybridization. 

 Plasticity may produce a similar effect. The colour of the 

 leaves of the copper beech is a compound property ; we call it 

 red or reddish, but in reality it is produced by the coexistence 

 of red and green. In the shade the primordium red becomes 

 latent and the green colour is isolated. 



Another interesting example is afforded by Deschampsia 

 {Air a) ccespitosa. According to the floras, this species grows 

 in moist, shady places and its spikelets are silvery-grey or 

 purplish. I have observed (Gatley, Wilmslow, Hazel Grove; 

 near Manchester, August, 1916) two forms which differ by 

 their facies. In moist, sunny places the spikelets are shiny, 

 purplish (or rather purplish-brown), the grey colour of the 

 glimies being concealed by a purplish-brown pigment. In 



1 These examples are taken from among a large number of similar cases. 



