QUANTITATIVE METHOD AND PRIMORDIA 43 



§ 38.— MENDELISM (continued). SEGREGATION OF 

 COMPOUND PROPERTIES INTO THEIR COMPONENTS. 

 COMPARISON BETWEEN MENDELIAN DISSOCIATION 

 AND SEGREGATION PRODUCED BY PLASTICITY AND 

 GRADATION AND IN THE COURSE OF INDIVIDUAL 

 DEVELOPMENT. Causes dissimiles, similes effectus. — When 

 two parental species differing in one pair of primordia D and R 

 are crossed, both possibilities exist simultaneously in the first 

 hybrid generation Fj and the property R is latent. In the next 

 generation F2 (in the typical cases) the property D is visible 

 in certain specimens and the property R in others, according to 

 the Mendelian rule of segregation (dissociation). For instance, 

 the cross blue flax x white flax gives in Fj blue, in Fg white and 

 blue. 



Segregation produced by plasticity. — On the other hand, when 

 we take the offspring of a certain blue Campanula and cultivate 

 some specimens D at ordinary temperature (15° to 20° C.) and 

 other specimens R about a temperature of 30° C, the D speci- 

 mens are blue, the R specimens white. In the offspring under 

 consideration plasticity has brought about segregation. The 

 result is the same, with regard to the visible properties, as if 

 a blue Campanula had been crossed with a white one (§ 16). 



When we divide one specimen of Philodendron pertusum 

 (characterized by perforated leaves) into two parts P and p, 

 and bring P into a warm, damp greenhouse and p into a green- 

 house which is cool and dry, the new leaves of P are perforated, 

 but in the new leaves of p the characteristic perforations are not 

 (or hardly) observed. Here, again, segregation is a consequence 

 of plasticity. The plants P and p differ in an observable 

 property, just as two specimens of the Fg generation produced 

 by a (supposed) cross between Philodendron pertusum and a 

 Philodendron with non-perforated leaves. 



The species Fagus silvatica includes two subspecies : the 

 ordinary green beech (leaves green) and the copper beech, 

 the leaves of which are reddish. When a copper beech is 

 grown in deep shade its leaves are green ; the reaction which 

 produces the red colour is only possible under the influence of 

 Hght.^ Between two specimens taken from a shady and a 

 sunny place the same difference exists with regard to the colour 

 as between two specimens of the F^ generation produced by a 

 (supposed) cross green beech x copper beech. In this example 

 of segregation produced by plasticity all possible transitory 

 colours are observed between the extremes green and reddish, 

 according to the variations in degree of light and shade, in the 

 same way as in certain examples of hybridization continuous 



> I call shade and light the whole of the conditions of life which prevail 

 in a shady or a sunny place. 



