i66 NATURAL SCIENCE. March, 



" The assemblage of migratory birds in large flocks, which in 

 many cases wait for a favourable wind before they venture to cross 

 wide stretches of sea, and consequently start altogether as soon as 

 the weather is suitable, and arrive on the other side in enormous 

 numbers or rushes ; the keen sight of birds and their extraordinary 

 memory for locality ; the great variety of routes chosen, and the 

 pertinacity with which each species keeps to its own route — these and 

 many other facts all point in one direction. The desire to migrate 

 is a hereditary impulse, to which the descendants of migratory birds 

 are subject in spring and autumn, which has acquired a force almost, 

 if not quite, as irresistible as the hereditary impulse to breed in the 

 spring. On the other hand, the routes of migration have to be 

 learned by individual experience. The theory that the knowledge of 

 when and where to migrate is a mysterious gift of nature, the 

 miraculous quality of which is attempted to be concealed under the 

 semi-scientific term of instinct, is no longer tenable." 



The views here expressed appear to me to harmonise well with 

 the general conclusions as to the nature and limitations of instinct 

 arrived at by Professor Lloyd Morgan, and they are enforced by some 

 considerations which writers on this subject usually overlook. The 

 numerous recorded facts of birds returning year after year to build in 

 the same spots as in the preceding year, indicate that most of the 

 spring migrants are old birds. Not only is this the general belief of 

 observers, but it is rendered probable by the known longevity of most 

 birds, and the obvious circumstance that those which have escaped 

 the dangers of the double migration on the first occasion will be more 

 likely to escape in each succeeding year, so long as health and 

 strength continue. The fact that the breeding population of birds 

 in any country does not increase year by year, but, though there are 

 considerable fluctuations, remains on the average constant, proves 

 that there must be an enormous destruction of the young birds, which 

 certainly amount in number to several times as many as the old ones, 

 and it seems probable that this destruction takes place during the two 

 annual migrations, and more especially during the first one in autumn, 

 when the young birds have had no practice in long continued flight 

 and no experience of the dangers of the sea. If the birds of more 

 than one season live on the average only four or five years more, it 

 follows that only a very small percentage of the enormous annual 

 progeny of young birds can survive to take their place. Hence it may 

 well be that all those countless myriads of birds of the year that visit 

 Heligoland are among the failures which, if they leave the island, 

 perish in the waters. We know that enormous numbers 7imst perish 

 during each year, and where so hkely as during that first attempt to 

 traverse the North Sea ? This is rendered almost certain by the 

 recently issued Report of the British Association Committee on Bird 

 Migration, in which it is stated that at the various periods of the great 

 autumnal rushes at Heligoland, when countless thousands of birds 

 pass over that island, no corresponding influx has been noticed on our 

 east coasts during the four successive years that the two records have 



