'JO'J Phillips, I P&iodic A racy. [April 



periodicity to compare our time sense with. There occurs to one 

 immediately the phenomenon of ovulation and menstruation in 



man. and the appearance of rut in other mammals. But such 



appearances are notoriously liable to time error, and easily affected 

 by changed conditions, disease, season, etc. Variation in periods 

 of gestation and incubation are. it is true, extremely close to the 

 mean for the species, but these periods are coupled and timed by 

 very marked changes in circulation and metabolism, and are 

 scarcely comparable with the migrating period of the bird, which 

 has nothing but the very doubtful stimulus of the developing 

 glands to cheek ami control the migration once it has started, and 

 even this is sometimes not present at all. I use the word ' nothing' 

 as meaning nothing that we know of at present. It would indeed 

 be interesting if we could subject castrated birds to experimental 

 conditions in order to test the strength of their migratory impulses, 

 but this could hardly be accomplished. All we know is that young 

 birds belonging to migratory species, though they do not them- 

 selves breed during their first year, come north with the others, 

 perhaps by imitation. 



We acknowledge the dominating power of sexual 'hormones' 

 in instituting spring migration in birds, for the instinct of repro- 

 duction is the dominant instinct in animals, and all others may be 

 classed as secondary to it. What we cannot account for is the 

 controlling ami regulating power which must be constantly at work 

 once the journey is begun, in order that a certain latitude may be 

 reached at a certain time. 



For instance among warblers Cooke tells us ' that the Black 

 and White Warbler, an early migrant, occupies a whole month 

 in going from North Carolina to Massachusetts, averaging only 13 

 miles per day. while the BlackpoU Warblers that nest in Alaska 

 make the last part of their journey. 2500 miles, in not over two 

 weeks, or about 200 miles a day. Some Yellow Warblers accomplish 

 the last part of their journey to Great Slave Lake more than twice 

 as the average advance of spring over the saute region. 

 From New Orleans to Great Slave Lake they are continually meet- 

 ing colder weather. Fven the same species shows very different 

 rates of advance in different parts of the country. 



i Ctur - u R x ! 1907. 





