Recent Advances in the Study of Heredity. 277 
absence of Maple, and absence of Purple spot were associated in 
one individual. The actual colour of the seed-coat is a pale 
homogeneous grey, which becomes browner with age. 
Now it might be imagined that the hypothesis put forward to 
account for this case, the so-called Presence and Absence hypo¬ 
thesis, had only been elaborated to explain one or two outstanding 
difficulties. This indeed may have been its origin ; but its fate has 
been very different, for, as stated at the beginning of the lecture, 
this conception has supplanted the original form of it. 
Cotyledon Colour. 
Let us now therefore consider it in its application to the two 
pairs of characters of the cotyledons which we considered in the 
previous lecture. And let us deal with cotyledon colour first. 
Expressed in its simplest, and earliest form, the hypothesis as 
applied to this case is that the pair of characters is yellow and 
absence-of-yellow on a background of green, which, according to the 
hypothesis, is obscured by the yellow and can only be seen when 
the yellow factor is absent. This is really not much more than 
stating the case in other terms. In the case of the seed-coats one 
of the pair of characters might have been spoken of as Maple and 
brown, instead of Maple and absence-of-Maple ; for the seed-coat 
must be some colour, and the background which we see when the 
seed-coat is characterized by the recessive member of this pair is 
brownish. So that it may be said, that all that we are doing in 
applying the Presence and Absence hypothesis to cotyledon colour 
is to call the pair yellow and absence-of-yellow instead of yellow 
and green. And this is perfectly true. But there is a series of 
facts which have been observed by Mr. E. A. Bunyard which make 
it possible to base the application of this hypothesis to cotyledon 
colour on a more substantial foundation. 
By the application of certain chemical and photo-chemical 
tests, this observer brought the following facts to light. It is, 
probably, common knowledge that all Peas whether yellow or 
green when ripe and dry, are green when unripe. Such unripe 
Peas contain two pigments, a green one which is chlorophyll, and 
a yellow one which is a pigment of the xanthophyll series, always 
found in association with chlorophyll in the green parts of plants. 
The chlorophyll can be well seen in sections mounted in 
glycerine ; it gives the “ hypochlorin reaction ” with glacial acetic 
acid; and an alcoholic extract gives the characteristic bands in 
