132 How Animals Changed 



but will hatch on land as well as in water (Noble &' Brady, 1933) . 

 They will not hatch without considerable moisture, and tadpoks 

 usually emerge during rains. Salamanders of the genus Plethodon 

 have left the water during all stages in their life cycle. Their eggs 

 are to be found in moist situations under logs and stones or in holes 

 in the ground. The tadpole stage is passed within the egg mem- 

 branes before hatching. 



In Brazil, Lutz (1948) studied the life histories of frogs. She 

 found that the "general trend is toward withdrawal of develop- 

 ment from water, with concomitant changes in reproductive be- 

 havior, ecctopic adaptations, progressive increase of yolk volume 

 and acceleration of development." 



The chief reproductive adaptations which have enabled animals 

 to leave the ocean and take up life in fresh water and on land are 

 as follows: (1) Internal fertilization, which permits the union of 

 gametes without reference to the character of the external medium. 

 The union of gametes of many species takes place readily in fresh 

 water but is of course impossible in air. Many male animals in 

 marine, fresh-water, and land habitats produce spermatophores 

 which are left to be picked up by, or actually injected into, females 

 of the same species. (2) A shell, which protects and insulates — 

 conserves water and salts; protects from desiccation and injury. (3) 

 Food provision — as yolk in the egg cell, withimthe body of a par- 

 ent through such structures as placentae, or even by the feeding of 

 young, as by bees and birds. (4) Reduction in the number of young 

 and better care of them by such membranes as those about eggs and 

 the amnia of insects, birds, and mammals; by watchfulness and care 

 by parents of eggs or young. Care may extend even to the keeping 

 of the young warm by placing them in favorable situations or by 

 heat from the parent's body and to education for the work of adult 

 life. (5) Reduction of free-swimming larval stages and a general 

 tendency toward vivaparousness. Fresh water has less buoyancy and 

 f ree-swimmin| larval stages might be swept out to sea. There is little 



