STRUCTURE AND DEVELOPMENTAL RATE 249 



the free living individual. The lower vertebrates are almost 

 entirely aquatic and their eggs undergo only a short embryonic 

 development before reaching the swimming larval stage. The 

 birds and mammals, however, at the moment of birth or hatching 

 have, as a rule, attained a complexity of structure greater than 

 that of the adult stage in fishes and lower forms. The period of 

 their prenatal development is extremely long, offering far greater 

 opportunity in time for changes in the environment and, there- 

 fore, necessitating some means of control on the part of the parent 

 generation. 



The marine and fresh-water fishes live in a more or less homo- 

 geneous medium which rarely undergoes sudden or marked 

 changes during the spawning seasons. Their eggs are deposited 

 in the water in instinctively chosen places during definite times 

 when the conditions of oxygen and temperature are generally 

 favorable for the given species. This developmental environ- 

 ment may in unusual cases fail in one or all respects. The water 

 may become so stagnant as not to supply oxygen, or it may 

 suddenly become either too hot or too cold for the welfare of the 

 developing eggs, or in a dry season it may become evaporated or 

 carried off, allowing the eggs to dry. The instinct of the fish 

 helps to guard against such accidents, and the eggs are deposited 

 at a season when the temperature changes are least likely to be 

 harmful, and localities are chosen where the water is properly 

 supplied with oxygen and is sufficient in amount to escape rapid 

 drying. 



The higher land-living vertebrates have no such surroundings 

 in which to develop their eggs. In becoming terrestrial, these 

 animals must have evolved not only appendages for locomotion 

 on land, but also some means of controlling or providing an envi- 

 ronment in which their long embryonic developm'^.nt could take 

 place. 



The eggs of reptiles and birds, as is well known, are provided 

 with comparatively enormous amounts of food-yolk surrounded 

 by layers of other food and enclosed in protective membranes and 

 shell. These arrangements not only supply food, but insure a 

 moist environment essential to all development and permit free 



