Vol. XXII 

 1905 



Recent Literature. 3 2 7 



makes his opinions clear. While apparently not discarding the idea that 

 the spring movement is incited by the periodic activity of the reproduc- 

 tive organs, he seems hardly to adopt it; and cites the "demonstrations" 

 of Cooke and Clarke of "the importance of temperature as a factor," only 

 to say that "the remarkable uniformity of climatic conditions prevailing 

 in the Tropics makes it clear that we must look elsewhere for an explana- 

 tion of the departure of migratory species which winter in this zone." 



That the incentive is primarily physiological seems more and more to 

 be confirmed with the advance of our knowledge of the manner and con- 

 ditions of the movement. That temperature is a powerful regulating fac- 

 tor as regards the rapidity of the journey after the birds are under way, 

 the researches of Cooke and others may be considered as having unques- 

 tionably demonstrated. That warm and cold changes in the weather 

 respectively accelerate or retard the northward bound birds in their 

 spring journey is at least one fact in bird migration that is not open to 

 question. 



Respecting the impulse to migrate in spring, the following from a 

 paper on the ' Origin of Bird Migration ' by Mr. F. M. Chapman, published 

 some ten years ago (Auk, XI, Jan. 1894, pp. 12-17), ni «y well be recalled 

 in this connection : "Many animals," he says, "have an instinctive desire 

 for seclusion during the season of reproduction .... Many species of 

 tropical sea-birds resort each year to some rocky islet, situated perhaps 

 in the heart of their habitat, where they may nest in safety. This is not 

 migration in the true sense of the word, but nevertheless the object is the 

 same as that which prompts the Plover to migrate to the Arctic regions, 

 and, be it further noted, the movement is just as regular. These sea-birds 

 pass their lives in the tropics, their presence or absence in anv part of 

 their range being largely dependent upon the food supply. But, as in 

 the case of a Warbler which nests in Labrador, they are all affected at 

 nearly the same time by an impulse which Urges them to hasten to a cer- 

 tain place. This impulse is periodic and is common to all birds ... .It is 

 evident, therefore, that external conditions have not created this impulse, 

 though it is possible that in many instances they may have governed its 

 periodicity. On the contrary, its causes are internal. In the case of the 

 sea-birds, for example, dissection will show an enlargement of the sexual 

 organs and it is this physiological change which warns the birds that the 

 season of reproduction is at hand " 



Under ' Migration Routes,' and ' How do Birds find their Way ? ' the 

 author's remarks presents little that calls for comment, being for the 

 most part a brief reference to more or less generally accepted views. He 

 is inclined, however, to combat the idea that birds either follow, or are 

 guided in their journeys by, physical features. He says : "Where physi- 

 cal features are followed, we may be sure it is not from the guidance they 

 afford, but because they mark out convenient highways." This assertion 

 appears to be based on what "the Swallow" does in England; from 

 which he concludes: "In the great masses of land, the guidance afforded 



