174 SECTIONAL ADDRESSES. 



control not only the large constant morphological features, but funda- 

 mental reactions such as those determining the condition of physio- 

 logical equilibrium and vitality in general. In so far as any distinction 

 can be drawn between the behaviour of factors determining the varietal 

 as opposed to the specific characters of the systematist, Heribert- 

 Nilsson concludes that the former are more localised in their action, 

 while the latter produce more diffuse results, which may affect almost 

 all the organs and functions of the individual, and thus bring about 

 striking alterations in the general appearance. S. caprea, for example, 

 is regarded as the reaction product of two distinct factors which together 

 control the leaf-breadth character, but which also affect, each separately 

 and in a different way, leaf form, leaf colour, height, and the periodicity 

 of certain phases. We cannot, however, draw a hard-and-fast line 

 between the two categories. The factor controlling a varietal charac- 

 teristic often produces effects in different parts of the plant. For 

 example, the factors which lead to the production of a coloured flower 

 no doubt also in certain cases cause the tinging seen in the vegetative 

 organs, and affect the colour of the seed. Heribert-Nilsson suggests 

 that fertility between species is a matter of close similarity in genotypic 

 (factorial) constitution rather than of outward morphological resem- 

 blance. Forms sundered by the systematist on the ground of gross 

 differences in certain anatomical features may prove to be more akin, 

 more compatible in constitution, than others held to be more nearly 

 related because the differentiating factors happen to control less 

 conspicuous features. 



Turning to the consideration of the more complex types of inheri- 

 tance already referred to, we find numerous instances where a somatic 

 character shows a certain degree of coupling or linkage with another 

 perhaps wholly unrelated character. This phenomenon becomes still 

 further complicated when, as is now known to occur fairly frequently, 

 somatic characters are linked also with the sex character. The results 

 of such linkages appear in the altered proportions in which the various 

 combinations of the several characters appear on cross-breeding. 

 Linkage of somatic characters can be readily demonstrated in the garden 

 Stock. Some strains have flowers with deeply coloured sap, e.g., full 

 red or pmrple ; others are of a pale shade such as a light purple or 

 flesh-colour. In most commercial strains the ' eye ' of the flower is 

 white owing to absence of colour in the plastids, but in some the plastids 

 are cream-coloured, causing the sap colour to appear of a much richer 

 hue and giving a cream 'eye.' Cream plastid colour is recessive to 

 white and the deep sap colours are recessive to the pale. "When a 

 cream-eyed strain lacking the pale factor is bred with a white-eyed 

 plant of some pale shade, the four possible combinations appear in 

 p. but not, as we should expect in the case of two independently 

 inherited one-factor characters, in the proportions 9:3:3:1, with the 

 double recessive as the least abundant of the four forms. We find 

 instead that the double dominant and the double recessive are both 

 in excess of expectation, the latter being more abundant than either 

 of the combinations of one dominant character with one recessive. 

 The two forms which preponderate are those which exhibit the 



