GEOGRAPHICAL DISTRIBUTION 



357 



second barrier, that between the South Atlantic and the South Pacific, is situated at a Hne 

 approximately level with Buenos Aires on the South American coast and passes in a 

 south-easterly direction to the region of the West Wind Drift. It is formed by the 

 junction of the Brazil current running down from the north and a cold current flowingup 

 from the region of the Magellan Straits. Here again there is a considerable difference in 

 temperature between the two currents, nearly twice as great as that between the South 

 African currents, and the fact that no South Atlantic species pass this barrier might be 

 taken to mean that temperature was the deciding factor. But to my mind, a more im- 

 portant factor than temperature is the relation of the currents forming the two barriers, 

 namely, that in the South African barrier the currents run parallel with each other, and 

 though in opposite directions, are not opposed, while in the South American barrier 



Fig. 46. Illustrating the two closed circulatory systems of currents in the southern hemisphere, in relation to 



the distribution of sponges in that area. 



they are opposed. In the first case, the mixing of the waters at the line of junction 

 allows the migrating sponges to be carried from the southerly directed Agulhas current 

 into the northerly flowing Benguela current, and thence shortly to be restored to a third 

 current, the South Equatorial, in which the temperature is similar to that of the Indian 

 Ocean currents, whence they have come. Once in the South Equatorial current, their 

 passage to the West Indies and the coast of Brazil is only a matter of time, and we do, 

 in fact, find that one at least of the species common to the West Indies and Australia 

 is also found on the coast of Brazil. 



Assuming that the distribution of sponges is the result of transport by ocean currents, 

 we should expect to find that the fauna north of Buenos Aires differs markedly from the 

 fauna south of that line, and so far as our limited knowledge of these regions can show, 

 this is actually the case. Unfortunately, we know very little of the faunas of these two 

 regions, and future research may conceivably prove the contrary, but from a rapid 

 examination made recently of a collection extending over these two areas this does not 



D VI 16 



