INTRODUCTION. 



19 



single direction of the compass, and that the birds usually do not travel in the region 

 lying between these high-roads. He furthermore demonstrated that the routes of the 

 water-birds chiefly follow the coast, or, where they cross the continents, along the large 

 inland watercourses, and admirably mapped the Old World routes of the " littoral mi- 

 grants," as he termed them, the preceding chart (Fig. 6.) giving an idea of the plan. 



Lookino- at this map, two features strike us at first as difficult to understand, viz., 

 the distinct routes across the open ocean, for example, the routes A, B, and X, as 

 also the crossing of the Mediterranean at certain points, which, besides, are not always 

 the shortest distance between the two continents. We might also think it strange that 

 marine birds should go inland as indicated by the routes C and D. 



In order to explain this, we have again to go back to an earlier geological period, 

 in fact, to the time when the 

 migration originated. In re- 

 gard to the first kind of routes 

 those across the open 

 ocean we can do nothing 

 better than transcribe Wal- 

 lace's remarks, which are as 

 follows : 



"Migrations of this type 

 probably date back from at 

 least the period when there 

 was continuous land along 

 the route passed over ; and it 

 is a suggestive fact that this 

 land connection is known to 

 have existed in recent geo- 

 logical times. Britain was 

 connected with the continent 

 during, and probably before 

 the glacial epoch, and Gib- 

 raltar, as well as Sicily, and 

 Malta, were also recently un- 

 ited with Africa, as is proved 

 by the fossil elephants and 

 other large Mammalia found in their caverns, by the comparatively shallow waters still 

 existing in this part of the Mediterranean, while the remainder is of oceanic profundity, 

 and the large amount of identity in the species of land animals still inhabiting the 

 opposite shores of the Mediterranean. The submersion of these two tracts of land 

 would be a slow process, and from year to year the change might be hardly percep- 

 tible. It is easy to see how the migration that had once taken place over continuous 

 land would be kept up, first over lagoons and marshes, then over a narrow channel, 

 and subsequently over a considerable sea, no one generation of birds ever perceiving 

 any difference in the route." 



The distribution of land and water, as alluded to by Wallace, is indicated on the 

 accompanying sketch-map (Fig. 7.) by the dotted lines which represent the 100 and 

 500 fathom lines; a comparative glance upon the two maps obviates any further expla- 

 nations of the routes A, 13, and X. The conclusion is obvious that the oceanic routes 



FIG. 7. Diagram showing the depth of the seas surrounding Europe. 



is the 100 fathom line; is the 500 fathom line. The 



areas on the present land included within the dotted line were sub- 

 merged at no time during the glacial period. 



