62 Causes of Emigrations 



ing beds, most species take no food and die aftef they have shed 

 their gametes. "The stimulus to the performance of the spawning 

 migration is the developing and ripening of the generative organs, 

 and the elaboration of an internal secretion from the ovary or testis 

 which produces an intoxication, and impels the fish to seek water of 

 definite physical conditions. What these conditions may be depends 

 on the former history of the species — the ^historical basis of acting' 

 being the determining factor in this choice" (Johnstone, 1908) . A 

 species' area of reproduction may differ widely from that of its gen- 

 eral distribution. An animal's requirements are often quite differ- 

 ent at different periods in its life cycle. 



In tropical America there is a swamp snail which always crawls 

 out of water and lays its eggs, like piles of rosy pearls, in bunches 

 on the stems of emergent plants. There are several species of frogs 

 which in similar fashion deposit their eggs in frothy masses so that 

 the young tadpoles when hatching will fall into the water below. 

 The terrestrial marbled salamander long before rains leaves clutches 

 of eggs on land under logs and around litter, choosing situations 

 such that they may be submerged before they hatch (Noble & 

 Brady, 1933). Some terrestrial salamanders have progressed fur- 

 ther in their adaptation to terrestrial existence. They pass through 

 all larval stages within egg membranes and are ready for life on 

 land when they emerge. 



In progressive adaptation from aquatic toward terrestrial life 

 there is a general tendency to eliminate free-swimming stages in the 

 life histories. Many marine gastropods hatch as swimming larvae; 

 pond snails develop within the jelly deposited by their parents; land 

 snails in logs or soil commonly deposit eggs which are surrounded 

 by tough shells, and these hatch into little snails much like their 

 parents. Decapod crustaceans commonly carry their eggs attached 

 to the abdominal appendages. Gravid females of some littoral spe- 

 cies run in and out of the water. Some fluviatile and terrestrial 

 crabs do not return to the ocean to breed, but have become ad- 



