438 SCIENCE PROGRESS 



leave the Pea for illustration. If we ask a corn-dealer for a 

 pennyworth of Maple or Partridge Peas, he will give us a kind 

 of pea the seed-coat of which at first glance appears a uniform 

 brown, but on closer inspection is seen to have a ground-colour 

 of a pale brown, on which is a very beautiful mottling, consisting 

 of anastomosing tracts of a rich brown colour. This type of 

 coloration is called mapling. Another type of coloration is that 

 which is often seen on (though it is not necessarily associated 

 with) the kind of pea which is much grown and eaten on the 

 Continent, and is known as the Sugar Pea. In this type of 

 coloration there are minute spots (discernible by the naked eye) 

 of slightly varying sizes and of a rich purple colour on a 

 greenish grey background. The former type will be referred 

 to as "maple" and the latter as "purple spot." 



When a pea with a maple seed-coat is crossed with one with 

 a purple spotted coat, the result is a pea on whose seed-coat 

 both mapling and purple spots exist. This suggests that maple 

 and purple-spot are [not allelomorphic to one another, but 

 belong to separate pairs which are supposed to be maple (M) 

 and not-maple (nM), and purple spot (P) and not-purple spot 

 (nP). This sounds very much like a logical exercise, a matter 

 of words and not of things. But the reality of it is shown by 

 breeding from the hybrids ; for by doing this we actually get 

 peas, which we can touch and see, which exhibit neither mapling 

 nor purple spotting. We get, in fact, the following four t3-pes 

 of peas in the proportions given by the numbers which precede 

 them : 



9 M P, 3 M, 3 P, I grey. 



If we write this in a form analogous to — 



9YR, 3YW, 3GR, GW- 



we should have : 



9 M P, 3 M nP, 3 P nM, i nP nM. 



The nP nM is grey because all these colours are on a back- 

 ground of pale grey. This proportion shows that we are dealing 

 with two independent pairs of characters, viz. : 



1. Maple and not-maple. 



2. Purple spot and not-purple spot. 



Now we come to those cases in which one of the characters 



