OCR Text |
Show 1 9 0 3 . ] GEOGRAPHICAL DISTRIBUTION OF STIDEltS. 357 differences in extremest cases being merely accorded subfamily rank and in others only generic rank, it is hard to believe that the ancestors of the existing fauna entered South America from North America in the early part of the Secondary Period, that is to say in pre-Jurassic times, and that they have been isolated since that date until the close of the Miocene. Again, if it be supposed that they passed southwards from the north after the union of the two Americas with the close of the Miocene, the conclusion is inevitable either that the wealth of genera now existing in South America has been evolved since that date, or that there has been such a wholesale destruction of genera in the north as to leave but one genus (Aphonopelma) in the Southern States of N. America at the present day \ The occurrence of this genus in the area in question may be equally well attributed to migration of its ancestors northwards into the Sonoran Region after the union of North and South America at the close of the Miocene. Again, since no members of the Aviculariin?e occur at the present time in Australia or Austro- Malaysia, it is needless to look to this area as the source of the South-American fauna of this subfamily. Africa, therefore, and the Mediterranean area alone remain as the centres whence the incursion could have taken place. In support of the view that preference in this connection should be given to Tropical Africa may be urged the following facts. The Aviculariinae of this region are confined to the forest-region of West Africa, which is roughly conterminous with the basin of the Congo and includes also the forest-covered district to the north of the Gulf of Guinea, and are unknown in South Africa. Secondly, the West-African genus Scoclra is apparently the nearest living ally of the Brazilian genus Avicularia; and Ileterothele and Solenothele, from the same region as Scoclra, are equally nearly related, especially the former, to the Patagonian Mitothele. Perhaps also some significance must be attached to the circumstance that the stridulating-organ which attains such a state of perfection in the Eumenopliorinae is represented in an imperfect and unspecialised state in many of the South-American genera of Aviculariinae. Summary o f the prfoeclmcj payes* In very early Tertiary times, or, perhaps, still earlier, the primitive Macrotheline Dipluridae arose in Eastern Asia and spread thence in four directions :- (1) South-eastwards into Australia and New 1 Three other genera, namelj' Ehechustica, Tapinauchenius, and Avicidaria, have been recorded from the Southern States of North America; the first from Texas, the second from Texas also and ‘ Indian Territory.' and the third from California. The generic affinities of the first are doubtful; it may indeed be referable to Aphonopelma. The records of the second and third have little value, since the specimens determined were immature females. |