THE ORIGIN OF SPECIES 



THEORIES OF DISTRIBUTION 



Several comprehensive theories of distribution have been proposed, fre- 

 quently with the solution of these difficult cases as a primary objective. 

 Theories to be discussed below include the continental drift hypothesis 

 of Wegener, the age and area theory of Willis, and the climate and evo- 

 lution theory of Matthew. 



Continental Drift. The continental drift hypothesis is generally asso- 

 ciated with the name of Wegener, although the idea was not strictly 

 original with him. It has been modernized and corrected by Du Toit. 

 In brief, the theory holds that from the Paleozoic Era until late in the 

 Mesozoic Era there were only two major land masses, Gondwana and 

 Laurasia, and these were in contact at times. Gondwana centered around 

 the South Pole, while Laurasia overlapped the equator and extended well 

 into the northern hemisphere. During the Cretaceous, these masses frag- 

 mented to form the present continents, and these have since drifted apart, 

 very slowly, toward their present positions. Gondwana gave rise to the 

 southern continents, Africa, South America, Australia, and Antarctica, as 

 well as to the Arabian and Indian Peninsulas and the major Pacific islands, 

 such as Madagascar and New Zealand. Laurasia broke up into North 

 America and Eurasia. There has since been a general northward drift of 

 all of the continents and an east or west drift of specific ones, so as to 

 reach the present positions. As the drift is always assumed to have been 

 very slow, the continents must not have been widely separated until well 

 into the Tertiary period, if the hypothesis is correct. 



This hypothesis was originally proposed on the basis of some facts of 

 Paleozoic floras, such as the presence of fossils of tropical and semitropical 

 plants in Alaska. A generally southern origin of the continents would read- 

 ily explain this. However, there are some serious difficulties too. The geo- 

 logical evidence for continental drift is rather scant, consisting mainly of 

 reciprocal curves of coast lines of lands which should have been rent apart. 

 For example the outline of the east coast of South America can be fitted to 

 the west coast of Africa in jigsaw puzzle fashion. However, the correspond- 

 ence of geological strata is much less convincing, and so geologists are 

 generally rather skeptical of the continental drift hypothesis. From a bio- 

 logical viewpoint, the major difficulty is not that it cannot explain specific 

 distributions but that there is not enough to be explained. According to 

 the theory, the continents were substantially continuous until late in the 

 Mesozoic Era and could not have been widely separated until well into 

 the Tertiary period. This would mean that corridor connections were gen- 

 erally present late enough that a very general exchange, such as actually 

 occurred across the north Pacific corridor, should have been world-wide. 

 As the various biogeographic realms arc actually quite distinct, it is diffi- 

 cult to believe that thev have licen iMoadly connected so recently. 



The Age anci Area Theory of Willis. The Age and Area theory of 

 WiUis was proposed on the basis of a very extensive study of plant geog- 

 raphy. Because a species must originate in a definite locality, and because 

 plants tend to enlarge their range by slow migration in all directions, 



S22 



