144 



THE AMERICAN NATURALIST 



[Vol. LII 



velocity of development increases more rapidly with in- 

 creases of temperature for the flounder than for the 

 plaice; the same difference exists between the whiting 

 and the cod. Krogh showed that the velocities of the dif- 

 ferent stages of the frog's egg, Fig. 1, are the same; but 

 the different stages in the life history of the sam.e animal 

 may differ in velocity at the same temperature. 



4. Dormancy.— Dormancy is of much importance among 

 animals inhabiting the same area. Thus the eggs of Eu- 

 branchipus and Diaptomus stagnates require both summer 

 drying and winter freezing before they will hatch. Dor- 

 mancy is common in the eggs of grasshoppers (Thomas, 

 79), walking sticks (Trouvelot), etc. Dormant periods 

 are common, occurring even in deer and armadillo em- 

 bryos (Patterson), and probably represent hereditary 

 remnants of impressions made on former generations by 

 seasonal rhythms. 



The causes of these rhythms often are simple. Con- 

 cerning delayed germination or dormancy of seeds, 

 Crocker and Davis ('14) have said: 



secured in a variety of ways: by almost absolute exclusion of water 

 by seed coats (as in the hard-seeded legumes and species of several 

 other families), by the limiting of the degree of swelling of the embryo, 



tion . . .-. and finally jSiaps by deficiency in salts. To this^ust be 



Dormancy has been overcome by drying in the case of 

 several species of insects in the writer's laboratory. 



III. Seasonal, Succession as Illustrated by the Spiders 

 of a Small Area oe Ground 

 In the spring and summer of 1910, Mr. G. D. Allen un- 

 dertook the study of the seasonal succession of the fauna 

 of an area in a vacant lot at Eighty-first Street and Black- 

 stone Avenue, Chicago, which is a pond in spring and low 

 prairie in summer, but did not complete the work, though 

 his collections were extensive and thorough, extending 



