REJUVENESCENCE 155 
which forms the framework of the chemico-physical 
basis of life. Chemically regarded, living means the 
co-ordination of characteristic chemical changes, 
differing in nature and in rate in different creatures 
and in different parts of the same creature; but as 
these proceed they form a colloid framework in 
which they work. As the complexity of the frame- 
work increases, and the stability of the organism, 
so, too, the effectiveness of its chemical reactions. 
Thus the earthworm is more of an agent than the 
amceba, and the bird more than the earthworm. 
But the nemesis is senescence—the tax on stability, 
and rejuvenescence is the attempt at evasion. 
In the Protozoa, as Weismann and others have 
pointed out, the recuperative processes are so perfect 
that natural death need never occur; the stable 
mortal parts of the colloidal substratum can be 
reduced and restored piecemeal, and the creature 
never grows old. It is probable that the same is 
true of a simple animal like the freshwater Hydra, 
which may also be credited with immortality. But 
as life became more worth living, and the organism 
more of an agent, it became more and more difficult 
for the processes of rejuvenescence, which are 
always going on, to keep pace with those of senes- 
cence, which, to tell the truth, may begin before 
birth! Then it became necessary to have special 
periods for rejuvenescence, “spring-cleanings”’ of 
the organism, and we see these in the de-differentia- 
tions and new beginnings associated with budding 
and the like, perhaps even in such striking phe- 
