PRESIDENT’S ADDRESS. 21 
cell-aggregate was originally formed, since it is that by which it is still 
produced, and we know that the life-history of the individual is 
an epitome of that of the species. Such aggregates were in the beginning 
solid; the cells in contact with one another and even in continuity : 
subsequently a space or cavity became formed in the interior of the 
mass, which was thus converted into a hollow sphere. All the cells 
of the aggregate were at first perfectly similar in structure and in func- 
tion; there was no subdivision of labour. All would take part in 
effecting locomotion ; all would receive stimuli from outside; all would 
take in and digest nutrient matter, which would then be passed into the 
cavity of the sphere to serve as a common store of nourishment. Such 
organisms are still found, and constitute the lowest types of Metazoa. 
Later one part of the hollow sphere became dimpled to form a cup ; 
the cavity of the sphere became correspondingly altered in shape. With 
this change in structure differentiation of function between the cells 
covering the outside and those lining the inside of the cup made its 
appearance. Those on the outside subserved locomotor functions and 
received and transmitted from cell to cell stimuli, physical or chemi- 
cal, received by the organism; while those on the inside, being freed 
from such functions, tended to specialise in the direction of the 
inception and digestion of nutrient material; which, passing from them 
into the cavity of the invaginated sphere, served for the nourishment 
of all the cells composing the organism. The further course of evolu- 
tion produced many changes of form and ever-increasing complexity of 
the cavity thus produced by simple invagination. Some of the cell- 
aggregates settled down to a sedentary life, becoming plant-like in 
appearance and to some extent in habit. Such organisms, complex in 
form but simple in structure, are the Sponges. Their several parts 
are not, as in the higher Metazoa, closely interdependent: the destruc- 
tion of any one part, however extensive, does not either immediately 
or ultimately involve death of the rest: all parts function separately, 
although doubtless mutually benefiting by their conjunction, if only 
by slow diffusion of nutrient fluid throughout the mass. ‘There is 
already some differentiation in these organisms, but the absence of a 
nervous system prevents any general co-ordination, and the individual 
cells are largely independent of one another. 
Our own life, like that of all the higher animals, is an aggregate 
life; the life of the whole is the life of the individual cells. The life 
of some of these cells can be put an end to, the rest may continue to 
live. This is, in fact, happening every moment of our lives. The 
cells which cover the surface of our body, which form the scarf-skin 
and the hairs and nails, are constantly dying and the dead cells are 
rubbed off or cut away, their place being taken by others supplied 
from living layers beneath. But the death of these cells does not 
