114 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



SUMMARY OF MAMMALIAN DISPERSAL* 

 HANS GADOW 



Australia as the earliest great mass of land permanently severed 

 from the rest is in almost undisturbed possession of the lowest mam- 

 mals. It is the sole refuge of the monotremes, and the marsupials 

 have narrowly escaped a similar fate. They take us to the next 

 independent continent, South America. This had three chances, or 

 epochs, of being stocked with mammals. Within the Cretaceous period 

 it seems to have received its marsupial stock from the north, the pro- 

 genitors of all modern marsupials. A second influx during the early 

 Tertiary brought edentates and rodents as its first Placentals from 

 Africa, and those queer Ungulates, the Toxodonts and Pyrotheria, 

 unless we prefer to look upon these Eocene extinct orders as truly 

 aboriginal to South America, when this was still continuous with the 

 ancient Brazil- Afro-Indian Gondwanaland. The third and last inroad 

 came once more from the north, when with the close of the Miocene 

 permanent connection with North America was re-established. This 

 brought the modern odd-toed and pair-toed Ungulates, with dogs, cats 

 and bears in their wake, and lastly man. 



There remains the huge North World. Eurasia and North America 

 have always formed a wide circumpolar ring, which repeatedly broke 

 and joined again. Whatever group of terrestrial creatures was 

 developed in the eastern, Asiatic, half, was sure to turn up in the 

 western, and vice versa. 



Lastly, the mysterious African continent. It began originally as 

 the centre of the ancient equatorial South World; it has lost these con- 

 nections and has become joined to the northland, after many vicissi- 

 tudes. It is therefore most difficult to apportion its fauna rightly; 

 moreover for fossils it is almost a blank, except Egypt. It must have 

 had some share in the evolution of mammals, like edentates, rodents, 

 insectivores, hyrax, elephants, sirenians and lemurs, all groups with 

 an ancient stamp. But what share it had, against Eurasia, in the 

 development of say ungulates, carnivores, monkeys, we do not know. 

 Not much is likely to have originated in Europe; the elephants, rhinos, 

 hippos, lions and hyaenas were migrants rather from than to Africa, 

 rarely across some Mediterranean bridge, usually by Asia Minor. 



The more dominant forms of our present fauna have originated, to 

 use an expression of Darwin's, "in the larger areas and more efficient 



* From Hans Gadow, Wanderings of Animals (1913), Cambridge University Press 



