xvi LIFE-HISTORIES 329 



one that is very perfect at birth. It may be objected 

 that most marine fishes are hatched as far from perfect 

 larvae, yet have to develop for a long time exposed to 

 risks much greater than those that beset the young 

 Cetacean. The answer is that this is only possible for 

 fishes because they are so prolific. Their race can stand 

 a prodigious juvenile mortality, unthinkable in the case 

 of the slowly reproducing uniparous whale. 



4. Intricate Life-histories. In many cases the life- 

 histories of animals are perplexing in their complica- 

 tions. 1 There is, for instance, the strangely circuitous 

 development of most Echinoderms. The newly hatched 

 diffusely ciliated larvae of sea-urchins, sea-cucumbers, 

 starfishes, and brittle-stars turn into extraordinarily 

 shaped forms adapted to free-swimming in the open sea. 

 After a time there begins within the larva a new forma- 

 tion, on a fresh architectural plan, utilising some of the 

 previously established parts and rejecting others, and a 

 miniature of the adult is formed. The wandering amce- 



O 



boid cells which play so diverse and so important a role 

 in the animal kingdom are very active, at once as sappers 

 and miners in breaking down the old, and as builders in 

 constructing the new. 



In the familiar case of gnats or mosquitoes, the eggs, 

 which are moored in little rafts to water-weed, hatch 

 into quaint dark-coloured larvae, with slender bristly 

 bodies, mouth-parts that waft in food-particles, and a 

 valved breathing tube at the end of the tail with which 

 they perforate the surface-film of the pool. They feed 

 and grow and moult, and at the fourth moult a pupa 

 emerges, light-brown in colour, with a large head and a 

 small body, w r ith two anterior breathing tubes and no 

 open mouth. After a few days the pupa husk splits and 

 the winged insect escapes into the air. There could hardly 

 be a more zig-zag life-history. 



Prolonged Youth. In higher animals, but most 

 notably among mammals, there is an interesting tendency 



1 A large number of life-histories have been described in the 

 chapter on " The Cycle of Life " in the author's Wonder of Life. 

 plelrose, London, 1914.) 



