Arabia, Madagascar, India, Australia, and Antarctica 

 (Wegener 1924, UuToit U\y). Tiiis theory is sug- 

 gested by tbe sliajjes of the continents that could con- 

 ceivably be fitted together ; by the characteristics of 

 the Atlantic Ocean basin that makes it appear to have 

 been formed by a rifting apart of land masses ; by the 

 similar geological stratigraphy shown by invertebrate 

 fossils in South Africa and South America : by some 

 similarities of present-day fauna and flora at the 

 same latitudes on different continents ; and by the 

 difficulty tropical species have dispersing over arctic 

 land bridges. Perhaps we should keep an open mind 

 towards this theory (\\'olfson 1955), but the weight 

 of present-day evidence, both geological and biologi- 

 cal does not strongly support it (Mayr 1952). 



Stability of continents 



Although the major land masses and ocean 

 depths have probably remained substantially un- 

 changed since life first originated, the continents have 

 been repeatedly flooded to various extents by inland 

 seas, elevated, and eroded. Continents now separated 

 have been connected by land bridges in times past : 

 one of the best known connected Asia and Xorth 

 .\merica at the site of the Bering strait. A land bridge 

 now connects Xorth and South America at the isth- 

 mus of Panama, and Eurasia and Africa are connected 

 by the isthmus of Suez. Land bridges may exist for 

 only a short time in the geological sense, but they 

 serve as important dispersal routes for those land ani- 

 mals and plants able to cross them. When a land 

 bridge allows free passage of most animals and plants 

 in either or both directions it is called a corridor 

 (Simpson 1940). If the land bridge is narrow, has 

 an unfavorable climate, a lack of suitable niches, or 

 too many competitors, it is called a filter — only a few 

 specially adapted species are able to pass over it. A 

 stveepstakes route of dispersal is one over which only 

 a few species pass, more or less by chance. Because 

 of its general unfavorableness, it is generally only one- 

 way. The island-hopping dispersal of organisms from 

 southeastern Asia through the south Pacific has been 

 accomplished by relatively few species ; the more dis- 

 tant the island, the fewer the species that have reached 

 it. 



Mountain barriers are not permanent ; in the 

 course of time they erode away. New mountains may 

 rise again in the same area or elsewhere. With such 

 changes in physiography come changes in climate and 

 vegetation as well. The explanation of present-day 

 distribution of animal groups requires a knowledge of 

 past changes in the geological history of the earth, as 

 well as in present day characteristics of continents, 

 oceans, and climates. 



A glance at a globe quickly shows that the con- 



tinental land masses are concentrated mostly in the 

 northern hemisphere. There are three broad land ex- 

 tensions southward below the e(|uator : the Malay 

 Peninsula, Kast Indies, and Australia ; Africa ; and 

 South America. The southern hemisphere otherwise 

 consists largely of vast expanses of oceans. On a land 

 route. North America intervenes -South America and 

 I'.urasia. Eurasia is the largest continent, is central to 

 all the others, and always has had a great diversity of 

 climate and terrain. The size, arrangement, and posi- 

 tions of the continents are of importance to interpreta- 

 tion of the jjast evolution and dispersal of animals. 



Considerable evidence (Matthew 1915) indicates 

 that some large groups of animals, notably mammals, 

 first evolved in Eurasia and then spread to other parts 

 of the world. North America was a less important 

 center of origin and dispersal. Periods of aridity and 

 glaciation are known to have occurred in the northern 

 continents during the Permian, at the end of the Tri- 

 assic, at the beginning and end of the Cretaceous, and 

 during the Pleistocene. Intervening warm, moist, uni- 

 form climates prevailed in the early Carboniferous, 

 Jurassic, mid-Cretaceous, and Eocene. 



During periods of continental emergence, climates 

 on the northern continents changed from moist and 

 warm to more arid and cold. Animals adaptively lim- 

 ited to moist, warm environments were restricted to 

 tropical regions or dispersed outward into the south- 

 ern land extensions. New, more advanced animal 

 types adapted to the new conditions in the north ap- 

 peared. Monotremes entered Australia at an early 

 date. Marsupials probably originated in Eurasia and 

 dispersed into Australia and South America during 

 late Mesozoic or early Tertiary, although there is no 

 fossil evidence for these suppositions (Darlington 

 1957). An opossum still occurs in North America, 

 but no marsupial is now present in Eurasia. Marsu- 

 pials did not get into Africa, but are present in South 

 America and have adaptively radiated into a variety 

 of forms in Australia. The predominance of marsu- 

 pials in Australia is probably due to their chance suc- 

 cess, at an early date, in surmounting the sweepstakes 

 route of islands from the Asian mainland. They 

 were the only placental forms in Australia until bats 

 and members of the rat family arrived, much later. 

 There have been successive waves of dispersal of 

 higher mammalian forms into Africa and South 

 America, but many recently-evolved mammalian types 

 are largely confined to the northern hemisphere, 

 where most of the primitive forms there have long 

 since become extinct. The origin and dispersal of 

 birds may have followed the same pattern as that of 

 mammals, but their geological history is less easily 

 traced because birds are not easily fossilized. 



A modification of Matthew's theory appears neces- 

 sary, at least for the cold-blooded vertebrates. Fresh- 

 water fishes, amphibians, and reptiles are most richly 



Dispersal, migration, and ecesis 151 



