THE CURVE OF LIFE 145 
period, lasting it may be for a couple of years, 
during which the fish makes its long journey, often 
of a thousand miles, from the open sea to the rivers 
and ponds, and there is a long youthful growing 
period, for several years, when energy is accumu- 
lated for the short strictly adult period which in- 
volves a return journey to the sea and ends appar- 
ently in death as the nemesis of spawning. 
Another type of life-history implies a suppression 
of the larval period and a lengthening out of the 
embryonic development within the egg-shell or egg- 
envelope. A good instance is to be found in a class 
of unfamiliar but widespread primitive animals, 
known as Onychophora, of which Peripatus is a 
representative genus. They are interesting syn- 
thetic types with certain features of insects and 
others of segmented worms; they are doubtless 
survivors from very ancient days, keeping a pre- 
carious foothold in many parts of the world in 
virtue of their shyness and their nocturnal habits. 
Now all these characteristically cryptozoic creatures 
are viviparous, and the young they bring forth are 
from birth miniatures of the adults. Moreover, 
the period of embryonic development is prolonged, 
sometimes lasting a year. Contrasting this with 
the life-history of a higher insect, we see that in 
the latter embryonic development is shortened to 
a minimum and the larval phase lengthened out 
extraordinarily. In birds the time that the parents 
can afford to spend in brooding is limited by the 
conditions of food supply, temperature, and the like, 
