47c 



THE AMERICAN NATURALIST [Vol. LII 



of the agent iself. The investigator of animals follows 

 the same plan. He likewise uses the same kind of meth- 

 ods in the investigation of the processes of functional 

 and structural development, not only as applied to the 

 actions of the agent, but to its origin as well, and thus 

 again we are justified in concluding that the process 

 method is of universal application. 



(b) Cycle of Activity.— The activity of agents is always 

 accompanied by the expenditure of energy. This expen- 

 diture does not take place at a uniform rate, there is a 

 pulsation, an ebb and flow, a rising and a falling. Periods 

 of activity are followed by periods of repose and a 

 rhythm is seen which can often be resolved into cycles. 

 The importance of determining such cycles has been well 

 expressed by Lockyer 3 as follows : 



cycle, and if that is not to be found in the temperate zone, then go to 

 the frigid zones, or the torrid zones and look for it, and if found, then 



it, and see what it means. It there i> no cycle, then despai/for a time 

 if you will, but yet plant firmly your science on a physical basis . . . and 

 having gotten such n bsi>i< n> this, wail for results. 



There are innumerable cycles in the responses of ani- 

 mals, and of these the life-history cycle is perhaps the 

 most generally recognized; but activity and response, 

 hunger and satiety, stimulation and response, are other 

 familiar expressions of these conditions. During these 

 cycles of change the relative amount of energy set free 

 varies greatly, in other words, its dynamic status 

 changes. As expressed elsewhere, I have stated ('15, 

 p. 10) : 



