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 types 



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 — 



9 YR, 3 YW, 3 GR, GW— 



Ave 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. : 



i. Maple and not-maple. 



2. Purple spot and not-purple spot. 



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



