Theory of continental drift 



The theory of continental drift postulates an original northern land 

 mass, called Laurasia, and a southern one, called Gondwana. Ac- 

 cording to this concept, each eventually broke into several segments 

 which eventually became the present continents. It is further assumed 

 that occasionally Laurasia and Gondwana drifted close to one another 

 or were at times in actual contact. On the basis of this geological 

 theory, Wolfson (1940) has attempted to explain the migrations of 

 some species of birds from one hemisphere to the other, as, for example, 

 the Greenland wheatear, Arctic tern, and several shore birds (turnstone, 

 sanderling, knot, golden plover, and others). Acceptance of this hy- 

 pothesis requires abandonment of the belief that the development of 

 migration was the result of useful ends that were served thereby, and in 

 its place, to give approval to the idea that migration was merely "the 

 natural consequence of an inherent behavior pattern responding to the 

 drifting of continental masses." 



It is a strange fact that although almost all professional paleontolo- 

 gists are agreed that existing data oppose the theory of continental 

 drift, those who support it contend that their case is strengthened by 

 these same data. If, in the geologic history of the earth, there was any 

 such thing as continental drift, it appears from the evidence available 

 that it was before the Cretaceous period, estimated to have been about 

 70,000,000 years ago. Birds had then evolved but those known from 

 fossil remains were of extremely primitive types such as Hesperornis 

 and Ichthyornis. There is no evidence of the existence in that period 

 of any birds that were even closely related to any of those now living. 

 Accordingly, it is difficult to believe that the migratory patterns of 

 existing species have been determined by events that, if they did take 

 place, were at least 70,000,000 or more years ago. 



When Birds Migrate 



It is known that at any given point many species leave in the fall and 

 return in the spring. Since banding has had such wide application as 

 a method of study, it is known also that in some species one of the 

 parent birds (rarely both) frequently returns and nests in the tree, 

 bush, or box that held its nest in the previous season. One ordinarily 

 thinks of the world of birds as quiescent during two periods each year, 



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