238 
A. D. Darbishire. 
to remember that the question we have to ask ourselves, in con¬ 
sidering the evidence relating to any thesis, is not “ how many facts 
support it ? ” ; but “ is there a single one which is against it ? ” 
Mendel’s hypothesis is a very simple one, and explains so large 
a number of otherwise anomalous phenomena, that we should from 
the outset adopt an extremely critical attitude towards it. 
Let us deal with the interpretation of the phenomena, part of 
the evidence for which we considered in detail. There is no 
observable difference between the germ-cell which, if it unites with 
its like, gives rise to a green Pea and one which, if it unites with its 
like, gives rise to a yellow. Yet we cannot but believe that there is 
something in the one which represents the green colour and some¬ 
thing in the other which represents the yellow. We may call these 
things for simplicity of expression the green-factor and yellow-factor 
respectively. 
Mendel’s theory related primarily to the condition of the germ- 
cells of the hybrids with respect to these factors. He supposed that 
half of them bore the green and the other half the yellow factor. He 
further supposed—and the burden of proof, of course, rests with 
anyone who supposes the contrary—that in fertilization these germ- 
cells mate at random, that is to say without reference to the colour- 
factors which they contain. The result, on these hypotheses, of the 
union of the germ-cells of two hybrids would be the followino four 
types of union, which would occur with equal frequency if the 
number of germ-cells concerned were a large one. (By G is signified 
a germ-cell containing a green factor; and by Y one containing a 
yellow). 
Y x Y = Yellow. 
Y x G = Yellow. 
G X Y = Yellow. 
G X G = Green. 
Now it is supposed that the pure-breeding yellow produces 
only yellows, when mated with its like, because it only contains germ- 
cells bearing the factor for yellowness, and that the pure green 
breeds true for a similar reason ; and we know, if this hypothesis is 
correct, that the result of the union of a germ-cell containing the 
yellow factor with one containing the green is a zygote (as the 
product of the union of two germ-cells is called) hearing the yellow 
character. Therefore the results of the four types of unions will be 
what are written to the right of them, namely three yellows and 
one green. But it will, of course, immediately be seen that of 
