20 Evolution and Adaptation 
and in other forms those parts of the body containing the 
eggs break off, and, after setting free the eggs, die. These 
are extreme cases of what is seen in many animals, namely 
the replacement of the old individuals by a new generation ; 
and while in general there is only a loose connection between 
the death of the individual and the consummation of its repro- 
ductive power, yet the two run a course so nearly parallel 
that several writers have attempted to explain this connection 
as one of racial adaptation. 
It has also been pointed out that in those higher animals 
that take care of their young after birth, the life of the 
individual does not end with the period of birth of the young, 
but extends at least throughout the time necessary to care 
for the young. It has even been suggested that this length- 
ening of the life period has been acquired on account of its 
use to the species. When, however, as in the case of the ver- 
tebrates, the young are born at intervals either in great 
numbers at a birth, as in fishes and amphibia, or in lots of 
twos, threes, or fours, as in many birds and mammals, or 
even only one at a time, as in a few birds and in man, it 
will be evident that the relation cannot be so simple a 
has been supposed. It cannot be assumed in these forms that 
the end of the life of the individual is in any way connected 
with the ripening of the last eggs, for, on the contrary, 
hundreds, or even many thousands, of potential eggs may 
be present in the ovaries when the animal is overtaken by 
old age, and its power of reproduction lost. 
In regard to several of the lower animals, we find, in a num- 
ber of cases where there are accurate data, that the individ- 
ual goes on year after year producing young. Whether 
they ever grow old, in the sense of losing their power of 
reproduction, has not been definitely determined, but there 
is, so far as I know, no evidence to show that such a pro- 
cess takes place, and these animals appear to have the power 
of reproducing themselves indefinitely. 
