77 8 



POPULAR SCIENCE MONTHLY. 



species collectively. The inside column refers to temperature. The 

 period of observation was twenty days, as shown by the line across the 

 top of the figure.* 



A, migration ; B, temperature. 



The advent of spring is marked by the northward progression of 

 the isotherm of 42.8° F., which is the initial temperature required to 

 awaken the dormant reproductive and germinating activities in ani- 

 mals and plants. "With the gradual invasion of the United States, 

 from the south northward, by temperatures above this, there passes 

 over the different regions the ever-old but ever-new panorama of the 

 spring with its opening blossoms, its unfolding green, and its waves 

 of migrating birds. The restlessness produced by the periodic devel- 

 opment of the reproductive function under the stimulus of increased 

 temperature causes the highly organized bird life to spread out from 

 its winter quarters, wherever those may be, and follow the zone of 

 new green that steadily widens northward with its increase of food 

 supply in the form of myriads of insects. The comparative regularity 

 in the recurrence of this phenomenon year after year is attested by 

 the observations just noted. Each species has a certain, definite 

 physiological relation to temperature, and its migratory movement 

 toward the breeding ground is determined by the movement of the 



* Stone. The Birds of Eastern Pennsylvania and New Jersey. 



