REJUVENESCENCE 153 
organs like brain and heart, liver and kidneys, for 
a chain is no stronger than its weakest link. Others 
have referred to the waning activity of the all- 
important organs of internal secretion, and others 
to the sobering fact that there is no multiplication 
or replacement of the cells of our central nervous 
system after a very early date in our life. It may be 
pointed out, however, that most, if not all, of the 
theories break down because they do not admit of 
all-round application (thus it is plain that many 
animals that are not troubled with a large intestine 
nevertheless grow old), and furthermore that the 
theories seize on symptoms rather than on causes; 
for while it is good sense to refer to wear and tear, 
the question arises why all animals do not exhibit 
the perfect recuperation to which some at least have 
attained. 
Professor Child has been working for fifteen 
years or more with simple creatures called Planarian 
worms, which he has come to know with con- 
siderable intimacy. One of the features of their 
life which he brought to light is their capacity for 
periodically becoming young again. Thus it often 
happens that a Planarian separates off the posterior 
third or quarter of its body, which speedily grows 
into a whole, while the diminished original heals 
itself and grows a new tail. This is an old story, 
but the new fact is that in the reconstitution of 
the whole from the part there is a remarkable 
process of rejuvenescence. Instead of the current 
of life being slow, it is very rapid; the output of 
