OCTOBER, 1902. 
THE MIGRATION OF BIRDS. 
of dispersion, or where on account of barriers these courses would 
be the only ones likely to be followed. Thus the wooded streams 
of our river valleys and the salt marshes of the coast line offer 
an abundant supply of food, and quite generally environmental 
conditions that are favorable to birds of every group. Coast 
lines furthermore present the ocean on one side as a vast barrier ; 
likewise island chains, while they present a favorable food sup- 
ply on the one hand, have the ocean to either side as a vast bar- 
rier. Mountain ranges, again, with their differepce in faunal 
areas, act as a barrier to certain groups, directing their move- 
ments along the more fertile foothills ; as an inducement to others 
since they are enabled to find their accustomed faunal areas wind- 
ing their way through other faunal areas that would be anything 
other than an inducement. 
It now becomes evident that birds will return to the proximity 
of their old homes even in the absence of home affection. 
The fact that some few birds seem to follow a different course 
in fall from the one followed in spring might at first appear to 
offer difficulties to the principle evolved above. If it be remem- 
bered, however, that migration originated as a result of disper- 
sion, and, in all probability of successive dispersions of the same 
species both from the north and the south, it is obvious that in 
the south different physiographic features may have influenced 
the birds from those that influenced them in the north, and so lead 
to a circular route. These cases must not be confused with those 
referred to by Prof. Baird, since in the latter the birds were al- 
ready extensive migrants, they were beyond the stage of the ac- 
quisition of the habit, and present us merely with a case of the 
extension of a migratory route under conditions already familiar 
to them. 
In connection with work of this nature two theories have 
been presented, at dift'erent times, regarding a sixth sense by 
means of which birds find their way between their summer and 
winter homes. . The first of these, that of Dr. A. von Middendorf, 
has already been shown to be a fallacy (Historical Review, pp. 
4-5, ante). The second is the result of careful investigations 
upon carrier pigeons. Capt. G. Reynaud, who is the author of this 
theory, has formulated what he terms a "law of retracement," or 
''law of reverse scent" (48). He says: ''When the time for de- 
parture is come, birds of the same species, inhabitating the same; 
region, come together for the journey. Those that have already^ 
48. Smithsonian Report, 1898, pp. 481-498. 
