Reproduction 135 



is supplied from food stored in the egg itself, but at times it is sup- 

 plied from other sources, such as secretions from a parent, or other 

 foods. A variable environment, if it is not too dry or too cold, tends 

 to hasten development (Shelford, 1929) . Land habitats, because 

 they vary more, in general are populated by animals which show 

 well-marked seasonal rhythms and often have very brief develop- 

 mental periods unless the young are retained within the body of the 

 parent or live in some special terrestrial habitat where conditions of 

 life are constant. For example, the tadpoles of highly terrestrial 

 amphibians have shorter lives than those which are more aquatic. A 

 spade-foot toad tadpole transforms into an adult in two to four 

 weeks after hatching, but a bullfrog tadpole lives for a year or two 

 (Wright, 1931). 



Reproductive and growth processes are more or less rhythmic 

 and are usually orderly and cyclic (Flattely, 1920; Brody, 1928; 

 Adolph, 1931a) . In stable, uniform environments, such as the open 

 ocean furnishes, animals are often erratic and perennial, but in the 

 littoral marine region, in fresh water, and on land they are usually 

 definitely correlated with seasonal successons. Hubbs (1928) be- 

 lieves that in fishes "structural differences between local races seem 

 largely the result of changes in developmental rate (and metab- 

 olism), not themselves of direct selectional significance. A har- 

 monious relation exists between the developmental rate and the 

 usual environmental conditions during growth. Normal develop- 

 ment is possible with some variation in conditions. But when a new 

 territory is populated, with, for instance, a lower temperature, the 

 individuals are brought closer to the threshold of abnormal develop- 

 ment, and a higher rate of elimination must result. A mutation 

 covering the developmental metabolism would adjust the popula- 

 tion to the new habitat. Such changes as an increase in segments 

 would secondarily result. The population would increase and push 

 on into still colder waters, where the same modifications would 

 be expected. A series of such primary physiological and secondarily 



