March, 1905 | 
IMMIGRATION ROUTES 
37 
as quoted above, it was worded in general terms, because the theory in its details 
is so well known — and moreover it was not done in an attempt to give an inde- 
pendent presentation of it but simply to apply it to a given ease. And yet I was 
careful to use the word “ indicate ” as I was not unmindful of the fact that there are 
cases which cannot be explaimed on this theory alone, whether the reason be that 
they are simply so great modifications that we are as yet unable to see through the 
the complication, or cases for which another theory must eventually be framed. 
Prof. Cooke at the outset calls attention to the “several species” which have 
different migration routes spring and fall and by the annihilating remark that 
“ evidently both routes cannot be the original path 0 of migration” he seems to think 
that he has refuted Palmen’s theory at least so far as these species are concerned. 
But, pray, why “evidently” ? It is quite thinkable , at least, that the two routes are 
simultaneous. Suppose, namely, that a species extended its range northward with 
a broad front along a wide stretch of land bounded east and west by the wide sea. 
It is conceivable that the climatic and food conditions were so different spring and 
fall on the two opposite coasts that it might have been highly beneficial for the 
bird to migrate alternately along the east and west shores, and I, at least, can see 
no impossibility in some migration routes originating in this way. On the other 
hand, one of the migration routes, probably the one in spring, may indicate the 
original way of immigration, while the other may be a much later modification. 
But now for the route of the prothonotary warbler and the route it follows. 
Prof. Cooke, in the article alluded to, says that it is known that those of the Missis- 
sippi Valley “pass neither to the west along the coast of Texas, nor to the east 
through Florida* but on arriving at the coast they make a flight across the Gulf of 
Mexico, here nearly at the widest.” He then goes on to show how he thinks the 
route once was further west at a time when the sea stood much higher and that 
the birds wandered along the coasts (then far inland) of Mexico and Texas; that 
as the land rose the birds straightened out the kink in the route and thus came to 
cross the Gulf where it is at the widest. He next makes the admission that others 
think the birds once migrated farther east, in the direction of Cuba, and later 
straightened out that kink by moving the route farther west. Apparently Prof. 
Cooke is willing to take either horn, for both “refute” Palmen’s theory. But this 
admission proves conclusively that Prof. Cooke does not know just where the 
route of the prothonotary warblers lies across the Gulf of Mexico once they are 
out of sight of land. In his “Distribution and Migration of the North American 
Warblers” (Bull. 18, Biol. Surv., [904) he plainly shows that the bird in question 
probably passes along the coast of Campeche, and also that during migrations it, 
occasionally, at least, touches Cuba and Florida, though he does not believe it 
passes through southern Florida, as it only becomes more numerous farther north. 
The great stumbling block in Prof. Cooke’s way apparently is the improbabil- 
ity of there having existed formerly a “chain of islands” from southern Mexico to 
the Mississippi, because of “the fact that the Gulf of Mexico off the mouth of the 
Mississippi River is avast abyss, with no indication that any of its central portion 
has been above water since bird life appeared on the earth.” 
This seems to be a very risky statement for a non-geologist to make, since it 
a Path is Prof. Cooke’s nomenclature. I spoke of the 'way” of immigration which in many cases undoubtedly 
involves a wide tract of territory. Birds might immigrate with a very broad front and yet they may migrate along 
comparatively narrow routes. It is essential to maintain this distinction. 
b Note well the difference between the expressions, “ along the coast of Texas” and "through Florida.” 
