March, x9o 5 I 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 case. 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 ?pecies" which have different migration routes spring and fall and by the annihilating remark that "evidently both routes cannot be the original path a of migration" he seems to think that he has refuted Palmdn'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 exteni:tedits 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 b 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" Palm?n'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" (Brill. xS, Biol. Surv., t9o4) he plainly shows that the bird in question probably passes along the coast of Campeche, and also that during nfigrations it, occasionally, at least, touches Cuba and Florida, though he does not believe it passes through southern Florida, as it only becomes more nunserous 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 a vast abyss, with no indication that any of its central portion has been above water since bird life ap[eared 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 "througa Florida."
