Vol i909" VI ] Trotter, Land-Bird Fauna of N. E. America. 225 



mainland by Northumberland Strait of a varying width of from 

 nine to thirty miles. There is evidence of considerable submergence 

 in the region about Amherst as Sir J. W. Dawson has shown in his 

 'Acadian Geology' (4th ed., 1891, pp. 29-31). Submerged forests, 

 mainly of pine and beech, have been found in several localities 

 about Cumberland Basin and Cobequid Bay, and the great 'dikes' 

 about the Basin of Minas, which are reclaimed maritime marshes, 

 Dawson regards as undergoing slow submergence. All these 

 facts indicate at least, an unstable condition, and taken together 

 with the low relief of the region as a whole and the present general 

 relations of land and water we are justified, I think, in believing 

 that this strait once existed, and that at a not very remote period. 

 Furthermore, there is much evidence to show that considerable 

 areas along the northeastern coast of the continent have suffered 

 submergence under the enormous weight of the ice mass that was 

 pushed seaward from the Laurentide Glacier. 



The relative distribution of land and water areas unquestionably 

 exerts an important influence in determining the range of various 

 species of land birds. Many land birds migrate over wide stretches 

 of sea, but, as Wallace has pointed out, such routes possibly indi- 

 cate a former land surface that has become gradually submerged. 

 The observations of Mr. Wells W. Cooke would seem to disprove 

 this, as the evidence he has gathered regarding the Gulf and Carib- 

 bean routes indicate that migrating birds frequently follow courses 

 that lead over the deeper parts of these waters. In the problem 

 before us, however, we are dealing with more than the purely 

 migratory impulse. This migratory impulse per se, I take it, is 

 the primitive instinct of certain species of birds to reach a northerly 

 region where food of a suitable kind for the young is abundant 

 and where the summer day is long, giving the maximum light 

 conditions under which to forage. 1 After a bird has reached this 

 summer home it will constantly tend to widen its breeding area, 

 spreading out over a larger territory, limited of course by various 

 ecological factors, as suitable habitat conditions, by the pressure of 

 other species, by the nature and abundance of food, by temperature, 

 and by the conformation of land and water areas. The entire 



1 See article by E. A. Sehafer, F. R. S., 'On the Incidence ot Daylight as a Deter- 

 mining Factor in Bird Migration.' Nature, Dec. 19, 1907. 



