Recent Advances in the Study of Heredity. 321 
explanation of a reversionary phenomenon by Mendelian theory ; 
in the second place it is one that can easily be repeated by anyone 
anxious to witness the facts for himself. There is, of course, no 
difficulty in obtaining a white-seeded Pea, and a field Pea with grey 
seed-coats can be obtained from or procured by any corn-chandler. 
This case also has a peculiar interest from the more general point 
of view of the interpretation of natural phenomena, in that it is a 
case in which we are not held up at one particular point where all 
we can say is that the theory put forward is consistent with the 
facts, and that we have no further demonstration than this that 
the theory approximates to the true explanation of them. In this 
particular instance all we can say when we have raised F 2 is that 
the theory put forward is consistent with the results obtained ; but 
we can proceed to discover whether or no this is a proof of the 
correctness of the theory by further experimentation with the four 
whites, as explained above. 
Again, the ratio 9:3:4 itself is interesting in more than one 
way. It will be seen that if we classify the GP and the Gp 
together as Peas with pigmented seed-coats, and refer to the white 
ones as unpigmented, there are twelve coloured to four colour¬ 
less, i.e., three coloured to one colourless. The procedure of 
lumping together the GP with the Gp is not a mere artificial 
association of two categories having nothing to do with one another 
for the purpose of obtaining a particular numerical ratio, but is 
justified by the fact that the Peas which we have described as grey 
do, as a matter of fact, bear very faint purple spots; so that the 
difference between GP and Gp is not the difference between a grey 
Pea which bears a purple spot and one which bears none, but 
between a Pea which bears a pronounced purple spot and one which 
bears a faint one. This fact is a curious one, when we consider it 
in relation to the theory of the gametic constitutions of the two 
types involved in the cross which has produced this result; for the 
faint (as it were, suppressed) purple spot, is borne by that of the two 
parents, namely the grey Pea, which carries the recessive member 
of the pair, purple spot and its absence, whilst the parent, namely 
the white Pea, which bears the dominant member of this pair, the 
factor for purple spot itself, exhibits no trace of this character 
whatsoever. And this fact would again suggest that the explanation 
of the phenomena which this cross presents may not be so simple as 
the Mendelian one which we have considered. 
There is another respect in which this ratio 9 : 3 : 4 ; is an 
