THE PRESIDENTIAL ADDRESS 17) 
which is in some sense higher than the chemical level. The main 
necessity from the standpoint of biochemistry is then to decide 
whether nevertheless at its own level, which is certainly definable, 
the results of experimental studies are self-contained and consistent. 
This is assuredly true of the data which biochemistry is now acquir- 
ing. Never during its progress has chemical consistency shown 
itself to be disturbed by influences of any ultra-chemical kind. 
Moreover, before we assume that there is a level of organisation 
at which chemical controlling agencies must necessarily cease to 
function, we should respect the intellectual parsimony taught by 
Occam and be sure of their limitations before we seek for super- 
chemical entities as organisers. ‘There is no orderly succession of 
events which would seem less likely to be controlled by the mere 
chemical properties of a substance than the cell divisions and _ cell 
differentiation which intervene between the fertilised ovum and the 
finished embryo. Yet it would seem that a transmitted substance, 
a hormone in essence, may play an unmistakable part in that 
remarkable drama. It has for some years been known that, at an 
early stage of development, a group of cells forming the so-called 
* organiser’ of Spemann induces the subsequent stages of differentia- 
tion in other cells. The latest researches seem to show that a cell- 
free extract of this ‘ organiser’ may function in its place. ‘The sub- 
stance concerned is, it would seem, not confined to the ‘ organiser ’ 
itself, but is widely distributed outside, though not in, the embryo. 
It presents, nevertheless, a truly remarkable instance of chemical 
influence. 
It would be out of place in such a discourse as this to attempt 
any discussion of the psycho-physical problem. However much 
we may learn about the material systems which, in their integrity, 
are associated with consciousness, the nature of that association may 
yet remain a problem. The interest of that problem is insistent 
and it must be often in our thoughts. Its existence, however, 
justifies no pre-judgments as to the value of any knowledge of a 
consistent sort which the material systems may yield to experiment. 
V. 
It has become clear, I think, that chemical modes of thought, 
whatever their limitation, are fated profoundly to affect biological 
thought. If, however, the biochemist should at any time be inclined 
to overrate the value of his contributions to biology, or to under- 
rate the magnitude of problems outside his province, he will do well 
sometimes to leave the laboratory for the field, or to seek even in the 
museum a reminder of that infinity of adaptations of which life is 
capable. He willthen not fail to work with a humble mind, however 
great his faith in the importance of the methods which are his own. 
