ANNIVERSAHY ADDRESS 195 



countries short of their destination, reach districts far 

 removed from their several points of departure. 



Far different is it with the Partial Migrants. By this 

 expression we denote those species of which, though the 

 majority leave the country under consideration, yet a 

 certain proportion seem to be resident throughout the 

 year. I say "seem," because in many instances what 

 appear to be partial migrants may prove to be true 

 migrants ; or, in other words, all the summer residents — 

 omitting exceptional ca'^es — may with one accord leave 

 their breeding quarters, while their places are supplied by 

 other individuals of the same species, which have been 

 bred in a colder climate. Of this tendency the Song 

 Thrush and Skylark afford well-known examples. Indeed, 

 it is perfectly possible that partial migrants are non- 

 existent, and that the above explanation holds universally. 



Lastly, thete are the Irregular Migrants, whose very 

 name implies that some exceptional circumstances have 

 prevailed upon them to alter their course of migration 

 to one entirely at variance with the normal, or even 

 forced them to migrate in the first instance. Such may 

 no doubt be ignored by the student of the ordinary lines 

 of migration ; nevertheless their actions are a matter of 

 the greatest inteiest to ornithologists, and may prove to 

 be governed by certain laws of periodicity, possibly them- 

 selves depending on periodicity in the weather of some 

 nature as yet unexplained, and on a consequent failure 

 in the food supply. At recurring intervals, for instance, 

 hordes of Pallas' Sand Grouse spread over Europe from 

 the Steppes of Asia; with much less regularity individual 

 members of certain American species pay their visits to 

 our shores, and even to those on the Continent; and, 

 at least in the case of the latter, we can hardly fail to 

 believe that the agency of stormy weather has contributed 

 to their appearance. 



Many other subsidiary points fall to the student of 

 migration to consider, and of these it may be sufficient 



