1899] DISTRIBUTION OF THE ARACHNIDA 227 



It is impossible to fix the date of the occupation of Australia by the 

 ancestors of these peculiar forms ; but if the existing Pedipalp and 

 Scorpion fauna of Indo- and Austro-Malayan spread over this archi- 

 pelago in the Pliocene period, as has been supposed, it is clear that the 

 ancestors of the Urodacidae must have entered Australia in pre-Pliocene 

 times. Hence it is possible that their entry synchronised with that of 

 the primitive marsupials, and this is believed by Mr Lydekker to have 

 taken place in the Eocene epoch. This author further believes that 

 the similarity between the existing Australian and the extinct 

 Patagonian Dasyuridae (Marsupials) is to be explained on the 

 hypothesis of a land connection in Tertiary times between Australia 

 and South America. This hypothesis would also account for the 

 occurrence in Australia of Cercophcmius, which belongs to a typically 

 Neotropical family. 



In Palaeozoic times it appears that the Scorpion and Pedipalp 

 fauna of North America and of Europe was practically identical, and, 

 judging from the available evidence supplied by the extinct mammals 

 and reptiles, it is permissible to suppose that the similarity extended 

 throughout the Jurassic and Cretaceous epochs. Moreover, during the 

 Tertiary period there must have existed between the eastern and 

 western portion of the northern hemisphere a northern land connection, 

 admitting a free interchange of the more northern representatives of 

 the fauna, at all events during the prevalence of sub-tropical conditions. 

 Nevertheless, even during the time when the northern union was com- 

 pletest and the conditions of temperature most favourable for an inter- 

 change of species, we should expect the fauna to become gradually 

 more and more distinct in the eastern and western moieties of the 

 northern hemisphere the farther to the south it extended. These 

 differences would become still more marked with the stoppage of all 

 intercommunication and the destruction of the northern forms with the 

 advent of glacial conditions in late Tertiary times. It seems, in fact, 

 unnecessary to look further afield for an explanation of the generic 

 differences that distinguish the scorpions, whip-scorpions, and Solpugas 

 of the Southern States of North America from the most northern repre- 

 sentative of these groups in Europe and Asia. 



This leaves the question of the origin of the fauna of South 

 America to be accounted for. 



It is held by geologists that South America was separated by sea from 

 the southern portions of North America in the Jurassic and Cretaceous 

 eras, and that the separation, which was probably continued through 

 the Eocene, came to an end by the end of the Miocene. If, then, 

 the ancestors of the existing Scorpions, Pedipalpi, and Solifugae 

 entered South America from North America they must have done so 

 either before the Jurassic epoch or during or after the Miocene. But 

 it appears to me that the structural differences between the South 

 American Arachnida and those of the rest of the world are not of 



