Studies on variation and selection. 



175 



by its presence in some combinations tends to make the eye black, 

 instead of ruby, and the factor e, which distinguishes black mice 

 from lilac ones does the same thing, does this analogous result suf- 

 fice to call these two genes "Polymeres" ? I think nobody would 

 want to call them so, because we know these genes as different, as 

 apart from eyecolour they both influence coat-colour, and this in a 

 different way. But then, must we call the genes studied by Nilsson- 

 Ehle Polymeres, simply because as yet we know only that they in- 

 fluence grain-colour, and because as yet we do not know, what other 

 possible effect each may have on the development of a wheat-plant? 



Looked at in this way, the distinction between Polyineres and 

 other genes becomes wholly artificial. 



There exists a good example, which beautifully illustrates this point. 

 In the Pea, de Vilmorin has found a gene which was absent from all 

 those pea-plants, of which the dry seeds adhere together in the pod. 



We will call this factor A. Plants with a never have any grains 

 sticking together. Plants without A may or may not present the 

 phenomenon, and this depends, apart from non-genetic influences, 

 such as a wetter or drier season, chiefly from the presence or ab- 

 sence of the genes we will call b, c, d, e, f, and G. In such a way, 

 that presence of d and G tends to make the grains cohere better, 

 and that presence of e and f makes them looser, so that they break 

 apart more easily. B and c finally, when they happen to be together 

 in the plant, have the same influence, but neither of these has by 

 itself any appreciable effect. 



This case would assuredly be classed as one of Polymerie, were 

 it not, that by chance we happened to know the six last genes, 

 apart from their tendency to favour or hinder the cohesion of the 

 grain. We know the influence these genes exert on other parts of 

 the plant. The presence of factors b and c together is necessary for 

 the formation of the waxy gloss on the whole pea-plant, seeds in- 

 cluded. This impalpable layer of waxy stuff seems to hinder the 

 ripening seeds in comenting themselves well together. 



The genetic factor d, by its cooperation or absence, makes the 

 difference between plants with horny, stiff, smooth pods and those 

 with soft pods, which cling to the seeds, and in drying often break 

 them apart. 



E makes the difference between white-flowered and rose-flowered 

 plants. The seeds of a plant with e are lightly coloured (if f be 

 absent), and have a lesser tendency to hang well together. 



