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, 



