94 THE WORLD OF LIFE 



group. This is due to several causes. In all extra-tropical 

 lands a large proportion of the species are migratory, and the 

 facts observed are very similar over the whole of the north 

 temperate zone. Some go to more northern lands in summer 

 to breed, returning south in autumn ; others leave us in autumn 

 to winter in the south, returning to lis in the spring; others, 

 again, are birds of passage only, staying with us a few days 

 or weeks on their way north or south. All these are con- 

 sidered to be truly natives, in our case to be ^' British birds." 

 But others only visit ns occasionally, some at very long inter- 

 vals, while others, again, are mere " stragglers," who have 

 lost their way or been driven to us by storms, and have only 

 perhaps been recorded (seen or killed) once or twice. There 

 is therefore a vast range for personal opinion as to what species 

 should or should not be included as " British " or " European " 

 or '^ Canadian " birds. If we add to this uncertainty the 

 extreme variety of opinion as to the limits of ^' species," " sub- 

 species," and " varieties," or ^' local races " of birds which 

 now exist, we see how hopeless it is to expect uniformity in 

 numerical estimates of the birds of different countries or re- 

 gions. As an example of this difference of treatment, we may 

 take two of the most recent estimates of the bird-population 

 of the world. Dr. Glinther, in 1881, estimated the species 

 of birds then known at 11,000, and Mr. Shipley added to this 

 an average of 105 new species per annum — estimated from 

 the y.oological Record — for the twenty-seven years elapsed 

 since that date, bringing the total up to 13,835. But in the 

 late Dr. Bowdler Sharpens Hand List of the Genera and 

 Species of Birds, just completed, the number is stated as being 

 18,937. This enormous divergence, as I am informed by an- 

 other great authority on Ornithology, Dr. P. L. Sclater, is 

 mainly, if not wholly, due to the fact, that Dr. Sharpe '^ in- 

 cludes as species all the numerous slight local forms which 

 are called ' sub-species ' by the new school of Ornithologists, 

 many of which, in my opinion, do not present sufficient dif- 

 ferences to require separation at all." 



