106 ON THE SPIDERS OF THE GENUS DESIS. [June 3, 



we possess as to the habits of these sea-spiders shows that they 

 live between tide-marks on the shore ; and that although gifted 

 with considerable activity on land, they are quite unable to swim 

 and are indeed practically helpless in the water. Hence their 

 pi-asence in S. Africa and S. Australia may be used as testimony 

 to the former extension between these countries, either of a 

 coast-line with shallow water, or at least of a submerged bank, 

 partially exposed at low tide. 



It may of course be urged by those who oppose the theory of 

 the former existence of an Antarctic tract connecting the southern 

 continents, that the species of this group of Spiders have reached 

 the coasts of Cape Colony, New Zealand, and Australia by 

 migration in a south-easterly and south-westerly dii'ection from 

 the coasts that border the Indian Ocean on the noi-th. This hypo- 

 thesis, however', affoi-ds no explanation of the fact that no spiders 

 of this genus have been recorded from any spot along the miles 

 of coast- line that intervene between, say, Durban and Singapore. 

 Nevertheless I was myself disposed to adopt the hypothesis of a 

 southward migration to account for the presence of these animals 

 on the coasts of the continents which separate the Indian from 

 the Atlantic and Pacific Oceans, until the example of D. 

 henyoncp. came to hand, to testify to the very close affinity between 

 the S. African and Australian types. If the hypothesis of 

 migration from the noi-tli be the sole explanation of the distri- 

 bution of the genus Desis, we should expect to find the annectant 

 form between the two types of structui-e exemplified by, say, D. 

 formidabilis fi-om S. Afi-ica and D. marina from New Zealand, not 

 in the south, but in the north. As a mattei- of fact it has turned 

 up in the south, and the most northei-ly known species, namely 

 D. TTiartensi from Singapoi-e, is no neai-ei" to the South African 

 type than is the species that occui'S in New Zealand. In my opinion, 

 therefore, the theory of a transoceanic land-connection supplies the 

 most satisfactory explanation of the affinity between the Australian 

 and South African species. Clearly, however, this theory does 

 not necessarily exclude the other entirely. The group may have 

 come down fiom the north in the first place, and subsequently 

 crossed, either from S. Africa to Australia, or the other way 

 about ; but until fui'ther light is thrown on the subject by the 

 discovery of species along the western and northern shores of 

 the Indian Ocean, I venture to think the available evidence 

 bearing on the question favours the view that the group originated 

 in the Austi'o- Malayan Islands and spread westwards from South 

 Australia to S. Africa, along a coast-line connecting the two 

 continents with one another. 



