A. D. Darbishire. 
318 
characters, whether of shape or of colour; but we have had no 
occasion actually to refer to it. When a cross is made between a 
Pea with this grey coat and one with a white coat, the hybrid has 
a grey coat with purple spots and is exactly similar to one of the 
two forms involved in the last cross we described. When these 
Peas are allowed to self-fertilise there is produced a generation in 
which a proportion which is entirely new to us obtains. It is, actually, 
composed of nine plants with purple spot on grey background, three 
with grey coats only and four with white. There are two features 
of this cross which are new to us, viz., the reversionary appearance 
of a character peculiar to the hybrid and possessed by neither of 
the parents, viz., the purple spots; and the production of the three 
forms in this new proportion 9 : 3 : 4 in F 2 . 
A theory has been put forward in explanation of these two 
results which is certainly consistent with what actually occurs. It 
is this. In this cross we are concerned with two allelomorphic 
pairs ; one is grey and absence - of - grey, and the other is 
purple-spot and absence - of - purple - spot; but it is a charac¬ 
teristic of the purple spot that the mere presence of the factor 
for purple spot in the fertilised egg-cell which gives rise to a Pea 
plant is not sufficient to bring about the appearance of the purple- 
spots in the seed-coats of that plant. The factor for grey coat must 
also be present. Two factors therefore are necessary to bring about 
the appearance of this purple-spot character in the zygote. It 
further follows that the factor for purple spot, that is to say the 
dominant member of the pair, purple-spot and not-purple spot, may 
exist in the zygote, without that zygote manifesting the purple 
spot; and the suggestion is that of the two forms involved in the 
cross we are describing, the grey Pea contains the factor for grey 
coat and the white Pea contains the factor for purple spot, 
but that this is not manifested because it is not associated with 
the grey coat. It will be seen from the application of this theory 
to this particular case that whilst the purple spot cannot exist 
without the grey coat the grey coat can exist without the purple 
spot; that is to say, whilst in this case two factors, only one of 
which can exist independently of the other, are necessary for the 
production of the purple spot, there are cases, as we shall see later, 
in which neither of the characters can exist independently of the 
other. But to return to the case before us. The grey-coated Pea 
contains the presence of grey coat and the absence of purple spot, 
whilst the white-coated one contains the presence of purple spot and 
