ANTARCTICA. 283 



living- forms are restricted to the Southern Hemisphere and part 

 of India — a composite area which would correspond to the 

 ancient Gondwanaland. Attempts have been made to explain 

 this by assuming that forms arose in the Northern Hemisphere 

 and radiated therefrom, so that the more primitive types would 

 he found furthest from the centre of origin, and that the latter 

 area would he peopled by more recent types. Such " polar 

 theories " do explain many distributions, but in many cases the 

 evidence of palaeontology gives anything but support. Further, 

 if such a theory be correct, then we Would have to assume some 

 general cause for the limitation of the area of origin to the 

 Northern Hemisphere. When we consider certain types, their 

 pa>t distribution, and the climatic conditions obtaining during 

 their time of origin, it is impossible to find any such general 

 cause. 



For example, it is well known that the Marsupials are 

 restricted at the present day to the Australian and South Ameri- 

 can regions; but that Marsupials abounded during Mesozoic 

 times in Europe and North America. Are we to regard their 

 present restriction to the Australian and South /\merican areas 

 as the result of the destruction caused in their ranks by the 

 evolution of higher forms of mammals in the Northern Hemi- 

 sphere? That such destruction did take place there is no doubt, 

 but strangely enough the fossil forms of the Northern Hemi- 

 sphere belong to a division known as the Polyprotodonts, which 

 live to-day in South America and Australia. The latter areas, 

 however, also possess forms known as Diprotodonts, which differ 

 from the Polyprotodonts in having a smaller number of incisor 

 teeth, and in the small size or absence of canine teeth ; and these 

 forms are unknown in the fossil condition in Europe and North 

 America. This is explained by the opposite school of thought 

 by assuming that a land bridge existed between Australia and 

 South America, and that this land bridge was Antarctica. 



The Polyprotodonts once universally spread over the lands 

 of the globe, became isolated from the other areas except South 

 America, and thus were protected from the more highly evolved 

 Mammals which destroyed the Marsupials of the northern areas. 

 This isolated Polyprotodont stock gave rise to Diprotodonts, 

 possibly in Australia, which migrated to the American area. 

 Although Diprotodonts are poorly represented in the latter area 

 at the present day, we are familiar with a number of fossil forms 

 from Patagonia. 



The occurrence of huge fossil land tortoises of similar form 

 in Patagonia and Australia supports such a contention. 



Were this the sole evidence for a land bridge between these 

 two continents, the argument based on the negative evidence of 

 the Northern Hemisphere might well be challenged. But we 

 shall see that there are many other distributions lending it sup- 

 port. 



Let us now consider a much more lowly group, which 

 includes the annulate freshwater and terrestrial worms, such as 

 the Earthworm. 



