PRESIDENT’S ADDRESS, 19 
on through the agency of the enzymes already mentioned, which are 
also of a colloid nature but of simpler constitution than itself, and 
which differ from the catalytic agents employed by the chemist in the 
fact that they produce their effects at a relatively low temperature. In 
the course of evolution special enzymes would become developed for 
adaptation to special conditions of life, and with the appearance of 
these and other modifications, a process of differentiation of primitive 
living matter into individuals with definite specific characters gradually 
became established. We can conceive of the production in this way 
from originally undifferentiated living substance of simple differentiated 
organisms comparable to the lowest forms of Protista. But how long it 
may have taken to arrive at this stage we have no means of ascertain- 
ing. To judge from the evidence afforded by the evolution of higher 
organisms it would seem that a vast period of time would be necessary 
for even this amount of organisation to establish itself. 
The next important phase in the process of evolution would be the 
segregation and moulding of the diffused or irregularly aggregated 
f nuclear matter into a definite nucleus around which all the 
Formation of : 8 : eateihes 
the nucleated Chemical activity of the organism will in future be 
cell. centred. Whether this change were due to a slow and 
gradual process of segregation or of the nature of a jump, such as 
Nature does occasionally make, the result would be the advancement of 
the living organism to the condition of a complete nucleated cell: a 
materia! advance not only in organisation but—still more important— 
in potentiality for future development. Life is now embodied in the 
cell, and every living being evolved from this will itself be either a 
cell or a cell-aggregate. Omnis cellula e celluld. 
_ After the appearance of a nucleus—but how long after it is im- 
possible to conjecture—another phenomenon appeared upon the scene 
in the o¢casional exchange of nuclear substance between 
Establishment cells. In this manner became established the process of 
of sexual : ~ : By 
differences. | 8€XuUal reproduction. Such exchange in the unicellular 
Protista might and may occur between any two cells 
forming the species, but in the multicellular Metazoa it became— 
like other functions—specialised in particular cells. The result of 
the exchange is rejuvenescence; associated with an increased tendency 
to subdivide and to produce new individuals. This is due to the intro- 
duction of a stimulating or catalytic chemical agent into the cell which 
is to be rejuvenated, as is proved by the experiments of Loeb already 
alluded to. It is true that the chemical material introduced into the 
germ-cell in the ordinary process of its fertilisation by the sperm-cell is 
usually accompanied by the introduction of definite morphological 
elements which biend with others already contained within the germ- 
cell, and it is believed that the transmission of such morphological ele- 
ca 
