PRESIDENTIAL ADDRESS. 387 
process. Each of the daughter-cells presently separates from its  sister-cell 
and goes its own way as a complete individual, still a Protozoon. It seems not 
improbable that the separation may be due to the renewed stimulus of hunger, 
impelling each cell to wander actively in search of food. In some cases, how. 
ever, the daughter-cells remain together and form a colony, and probably this 
habit has been rendered possible by a sufficient accumulation of surplus energy 
in the form of food-yolk on the part of the parent rendering it unnecessary for 
the daughter-cells to separate in search of food at such an early date. One of 
the forms of colony met with amongst existing Protozoa is the hollow sphere, 
as we see it, for example, in Sphzrozoum and Volvox, and it is highly probable 
that the assumption of this form is due largely, if not entirely, to what are 
commonly called mechanical causes, though we are not in a position to say 
exactly what these causes may be. The widespread occurrence of the blasto- 
sphere or blastula stage in ontogeny is a sufficiently clear indication that the 
hollow, spherical Protozoon colony formed a’ stage in the evolution of the higher 
animals. 
By the time our ancestral organism has reached this stage, and possibly even 
before, a new complication has arisen. The cells of which the colony is com- 
posed no longer remain all alike, but become differentiated, primarily into two 
groups, which we distinguish as somatic cells and germ-cells respectively. 
From this point onwards evolution ceases to be a really continuous process, 
but is broken up into a series of ontogenies, at the close of each of which the 
organism has to go back and make a fresh start in the unicellular condition, for 
the somatic cells sooner or later become exhausted in their conflict with the 
environment and perish, leaving the germ-cells behind to take up the running. 
That the germ-cells do not share the fafe of the somatic cells must be attributed 
to the fact that they take no part in the struggle for existence to which the body 
is exposed. They simply multiply and absorb nutriment under the protection 
of the body, and therefore retain their potential energy unimpaired. They are 
in actual fact, as is so often said, equivalent to so many Protozoa, and, like 
the Protozoa, are endowed with a potential immortality. 
We know that, if placed under suitable conditions, or, in other words, if 
exposed to the proper environmental stimuli, these germ-cells will give rise to 
new organisms, like that in the body of which they were formerly enclosed. 
One of the necessary conditions is, with rare exceptions, the union of the germ- 
cells in pairs to form zygotes or fertilised ova; but I propose, in the first 
instance, for the sake of simplicity, to leave out of account the existence of the 
sexual process and the results that foliow therefrom, postponing the considera- 
tion of these to a later stage of our inquiry. I wish, moreover, to make it quite 
clear that organic evolution must have taken place if no such event as amphimixis 
had ever occurred. 
What, then, may the germ-cells be expected to do? How are they going to 
begin their development? In endeavouring to answer this question we must 
remember that the behaviour of an organism at any moment depends upon two 
sets of factors—the nature of its own constitution on the one hand, and the 
nature of its environment on the other. If these factors are identical for 
any two individual organisms, then the behaviour of these two individuals must 
be the same. If the germ-cells of any generation are identical w'th those of the 
preceding generation, and if they develop under identical conditions, then the 
soma of the one generation must also be identical with that of the other.? 
Inasmuch as they are parts of the same continuous germ-plasm—leaving out 
of account the complications introduced by amphimixis—we may assume that 
the germ-cells of the two generations are indeed identical in nearly ever 
respect ; but there will be a slight difference, due to the fact that those of the 
later generation will have inherited a rather larger supply of initial energy and a 
slightly greater facility for responding to stimuli of various kinds, for the 
gradual accumulation of these properties will have gone a stage further. The 
environment also will be very nearly identical in the two cases, for we know 
from experiment that if it were not the organism could not develop at all. 
* This is, of course, a familiar idea. Compare Driesch, Gifford Lectures, 
1907, p. 214. 
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