170 PLANTS, SEEDS, AND CURRENTS 



abundantly near the sea, the pods are washed up in great quantities 

 on the beaches, and the freed seeds are to be seen germinating in 

 numbers, the seedlings striking into the sand. It occurs as a char- 

 acteristic beach shrub around the coasts of Hawaii, Oahu, etc., and 

 has been spread by the cattle far inland. Hibiscus tiliaceus is its 

 frequent associate. Spreading up some of the valleys of Oahu 

 Acacia farnesiana forms extensive thickets impenetrable for cattle, 

 typical chaparral scrub but of recent growth. This matter is dealt 

 with at length in my work on Plant Dispersal. 



The occurrence of Acacia farnesiana in oceanic islands needs a 

 little further consideration. One may suspect that this shrub or 

 small tree was introduced by man into islands when they lie far 

 from the continents, since its capacity for dispersal by the currents 

 would probably be limited to traverses of tracts of sea not more 

 than 400 or 500 miles across. Cambage ascertained that the seeds 

 preserved their germinative power after an immersion of five or 

 six months in sea-water (Journ. Proc. Roy. Soc. N. S. Wales, XLIX., 

 1915, p. 94). But since the seeds had no buoyancy, he appealed 

 to the pods. On finding, however, that a single pod sank in a few 

 days, he inferred that the seeds might be carried for very long 

 distances in drift-wood or pumice. This means of dispersal, how- 

 ever, would be at the disposal of all the species of Acacia that happen 

 to grow near the beach, and would not of itself be sufficient to explain 

 the exceptional range of the species in question. He does not 

 introduce the agency of man, an agency that would at once give 

 the plant a great advantage over other Acacias not so favoured. 



One can scarcely doubt that this plant was introduced by Euro- 

 peans at an early date into the Cape Verde Islands and Madeira, 

 either from the West Coast of Africa or from Southern Europe where 

 it has long been cultivated. It was collected by George Forster 

 on St. Jago in 1778 (1779?), and was regarded by Schmidt in the 

 middle of the last century as truly indigenous in the group {Flora 

 der Cap Verdischen Inseln, 1852, pp. 38, 342). Welwitsch, who was 

 in the islands about that time, refers to it as subspontaneous 

 (Catalogus Herbarii Gorgonii, by Prof. Coutinho, 1914-15). It also 

 seems to have been introduced into Fernando Noronha, though it 

 is now behaving as an indigenous plant. The thorny Acacia bushes 

 that were described by Moseley as abundant on the shore during 

 the visit of the Challenger about 1874 (Chall. Bot., III., 11) were 

 probably of this species. Ridley, who found Acacia farnesiana 

 growing in thickets in the interior of Fernando Noronha in 1887, 

 regarded it as having been introduced (Journ. Linn. Soc. Bot., vol. 

 27). In 1836 an Acacia was collected by Darwin on Keeling Atoll, 

 which was referred to this species (Chall. Bot., IV., 113). During my 

 sojourn on the atoll in 1888 I did not observe that it formed a feature 

 of the indigenous flora. It was probably one of the numerous plants 

 introduced in the early days of the occupation of the islands. 



Of the several authorities on the floras of the oceanic islands of 

 the tropical Pacific, not one includes this species amongst the indi- 

 genous plants, that is to say, the plants found in the islands at the 

 time of their discovery. Mann and Hillebrand for Hawaii, Seemann 



