PRESIDENTIAL ADDRESS, 679 
involved may turn out to be relatively simple, at any rate in their broader 
features. I mean that they probably belong to what we might term the lower 
grade of metabolic problems. For the great uniformity of the process as a 
whole, complex though it undoubtedly is, hardly suggests direct relations as exist- 
ing between it and those more specialised forms of metabolism on which the 
properties of specific form, and such like characters, depend. This view of the 
mutter is not in any way weakened by the fact that the materials providing for the 
multiplication of nuclei have themselves passed through the very highest 
stages of anabolic construction. There are, indeed, some grounds for believing 
that the composition of the higher proteins is distinctly spevific for different 
groups of organisms; but apart from this it is difficult to resist the conviction 
that, in so far as its essential constituents are concerned, the nucleus is the seat of 
a complex organisation which is superadded to its chemical composition. But 
this conception of the nucleus does not affect the position of the lower-grade 
chemical changes, with their physical accompaniments which are periodically 
rendered apparent during the rhythmical series of changes that culminate in the 
division of the nucleus. It is true that there are some who refuse to admit the 
necessity of what I might perhaps call architectural complexity in protoplasm. 
They prefer to regard all the phenomena of organisation and heredity as the 
outcome of dynamical, rather than of structural, conditions. It seems to me that 
it is impossible to reconcile such a view with the known facts respecting the 
inheritance of characters, and that we are driven to postulate the existence of 
material units which are together responsible for the sum of the characters 
represented in any individual. There are grounds for believing that these entities, 
whatever be their nature, are doubled, and are then equally distributed to the two 
daughter cells at every ordinary nuclear division; and thus the properties of 
organisation are preserved and transmitted over and above the flux of chemical 
change. 
Most people who have concerned themselves with cytological studies agree 
that the salient features of karyokinesis strongly emphasise the probability of a 
conservation of definite material; and that an extremely accurate distribution of 
it occurs where two daughter cells arise from a parent cell by division. And 
this inference is greatly strengthened by what occurs, more or less immedi- 
ately, in connection with the formation of the sexual cells. The origin of these in 
all the higher animals and plants, as is well known, can invariably be traced to a 
nuclear division of remarkable complexity. In this, the so-called heterotype 
division, the special feature consists in the sorting-out of the nuclear constituents 
originally furnished by the two parents of the individual. This sorting or dis- 
tribution takes place in such a way that each of the two daughter nuclei which 
arise as the result of the division receives only half the total number of chro- 
mosomes previously contributed by the two parents. The essential point of 
interest lies in the fact that the process does not consist in the mere halving of 
nuclear substance, but in the distribution of nuclear constituents. When two 
sexual cells which have been formed in this way unite to give rise to a new 
individual, the total number of nuclear chromosomes is again made good; but 
the resulting nuclear constitution will not exactly resemble that of either parent. 
That such is really the case is borne out by innumerable experiments that have 
been made by breeders. Furthermore the extensive investigations on the results 
of crosses, both in animals and plants, have confirmed the view that particular 
characters can be treated as entities. For they are distributed amongst the 
posterity of the original parents in proportions that closely approximate to mathe- 
matical expectation. In this distribution the separate characters behave inde- 
pendently. For instance, the green colour and round form of peas are two 
characters which may occur in the same or in different individuals. The nume- 
rical proportions in which they wild appear can be foretold with a considerable 
degree of accuracy. 
With these facts before us—and many others could be adduced, all pointing in 
the same direction—it is not easy to resist the conviction that within the nucleus 
there must exist material entities which are severally responsible for the appearance 
