1172 Life-histories of Northern Animals 



from which the species derives its name. Even before going 

 into winter quarters their soft silvery backs have given place 

 to the grizzly coats that characterize the adults." 



They are full grown by September; thus their very long 

 gestation is offset somewhat by a rapid growth to adoles- 

 cence. 



From sunset till black dark is the evening twilight, ap- 

 proximately equalled by the corresponding time before sunrise. 

 The duration of this half-light varies of course with each day 

 and at each latitude. In illustration Professor F. L. Blake, 

 of Toronto Observatory, has supplied me with the following 

 table of evening twilight at his station. Sunset is given in 

 standard time, 75thmeridian. Thetable is forlatitude43°,4o'N. 



March ist, Sun sets 6.05 p.m. Twilight lasts for i h. 39 m. 



April 1st " " 6.44 " " " " I h. 44 m. 



May 1st, " " 7.19 " " " " I h. 56 m. 



June 1st, " " 7.52 " " " " 2h. 24 m. 



July 1st, " " 8.03 " " " " 2h. 30 m. 



August 1st, " " 7.41 " " " " 2 h. 06 m. 



September 1st, " " 6.55 " " " " i h. 48 m. 



October ist, " " 6.00 " " " " i h. 41 m. 



The morning period is of about the same duration. These 

 two twilights are the time of the Bats, and all the evidence we 

 have goes to prove that it is the exception when they are 

 abroad in black darkness. 



It is probable that each species is adapted to a certain 

 measure of light, and those that come out early do not stay late. 



All must vary their habits with the ever-varying twilight, 

 but, broadly speaking, we can arrange the species in a scale 

 of crepuscularity, giving each an hour for its food-hunt and 

 leaving out the Say Bat, as not reliably observed. It is 

 worthy of note that the Bats come out nearly in order of size, 

 and that a similar scale and remark will probably be found to 

 fit our night-flying insects. 



