Recent Advances in the Study of Heredity. 173 
Let us now return to the Mendelian phenomenon. And let me 
here refer to a simple fact which, by reason of its very simplicity, 
is in danger of being overlooked, unless especial emphasis is laid on 
it at the outset. It is the fact that Mendel did two entirely distinct 
things; he observed and recorded the phenomenon which bears his 
name, and he put forward a hypothesis to account for it; I do 
not suggest of course that this fact is not perfectly patent to those 
to whom the spread of Mendelian theory has been due. 1 merely 
point out that in the majority of available expositions of this theory 
great stress is not laid on the precise point (which may not be so 
evident to the reader as, perhaps, at first sight it w uld seem that 
it should be) at which narration of phenomena ends and statement 
of hypothesis begins. One result produced by this on the mind of 
the student is that he inevitably receives the impression that there 
is as little doubt about the hypothesis as about the phenomena. 
The phenomena and the hypothesis become so intimately associated 
in the mind that the idea that the one stands or falls with the other, 
takes root. The result is that the upholders of the theory un¬ 
consciously come to regard it as as firmly established as the pheno¬ 
mena, whilst its critics are led, by their scepticism with regard to 
the hypothesis, to doubt the reality of the phenomena. A fact 
which has probably contributed largely to this close association in 
the mind between the hypothesis and the phenomena is that both 
of them can be and are represented by a singularly diagrammatic 
and simple scheme. But the two things are perfectly distinct; 
there is no question about the simple facts which I have narrated 
so far : I shall give the details of the evidence on which the state¬ 
ments as to the Mendelian phenomena presented by crossing yellow 
with green peas are based shortly. But although evidence favourable 
to the theory is being rapidly accumulated we should be rash indeed, 
in my opinion, if we were to regard the truth of the hypothesis as 
finally established. Another cause contributing to the association 
between these two things which it seems to me so important to keep 
apart, is that both take shelter under the one word Mendelism. 
I shall endeavour to emphasize the importance of keeping the 
two in separate compartments in the mind by not stating the 
hypothesis until the next Lecture and by not using the word 
Mendelism again. 
Let us now proceed to the consideration of the evidence for 
the phenomena which I have so far related. And in doing so I 
propose to confine myself to the evidence for the statement that, in 
