No. 614] PHYSIOLOGICAL PROBLEMS 



149 



Thus we come to the time of occurrence of dormancy 

 and the conditions necessary to break it up, which result 

 in the rhythmic tendency of the species fitting into the 

 rhythm of the climate in which it lives. In many insect 

 species it appears that drying may be substituted for 

 freezing. Such species may migrate into climates in 

 which there is a dry season, instead of a cold one, and 

 with a longer growing season, and continue with the usual 

 annual life-history rhythm. Under these conditions in 

 each growing season the development is stopped by dor- 

 mancy and proceeds no further until the drying breaks up 

 dormancy. The development of Eubranchipus, once ini- 

 tiated, proceeds until the mature individual has produced 

 eggs. Here dormancy stops all further progress until the 

 eggs are first dried and then frozen and warmed above 

 0° C. Crustacea without dormant periods go on devel- 

 oping and produce several generations in one summer. 

 After the conditions necessary for the overcoming of the 

 dormancy have been fulfilled, or where there is no dor- 

 mancy, species differ in the threshold conditions for de- 

 velopment. The thresholds for development are hardly 

 the same for any two species in which thresholds have 

 been determined. Thus species will differ in the time at 

 which development is initiated in the spring. Further, 

 the increase in velocity with increase in temperature is 

 different for different species, as indicated by the differ- 

 ences in the angle which their velocity curves make with 

 the axis of abscissas (see Krogh, '14, velocity curves 

 of several species of fish, also Fig. 2). This fact alone 

 makes it possible for a given set of conditions out of the 

 ordinary to give a peculiar and irregular occurrence of the 

 different species of a community. 



The total energy as illustrated by the G0 2 given off by 

 a species is the same for all conditions in which develop- 

 ment can occur at all, as shown by Krogh. It is probable, 

 accordingly, that the total energy expended in develop- 

 ment is different for each different species. This may 

 bear some relation to size and weight, though alcoholic 



