136 The Flora of the Galapagos Islands. 
and never attain great size. Many of the trees and shrubs are 
armed with spines or thorns, e.g., Mimosa , Acacia , Parkinsonia , 
Cereus, Opuntia, and Zantlioxylum . Climbers are neither so numer¬ 
ous or conspicuous as in the tropical parts of the American continent, 
moreover they are herbaceous, the woody liane so common in tropical 
jungles, being almost unknown upon the islands. The chief climbing 
genera are Boussingaultia , Cissampelos , Rhyncosia , Cardiospennum , 
Ipomoea , and three cucurbits, Elaterium , Momordica and Nicy os. 
Epiphytes, which occur only at the higher altitudes, are neither 
abundant nor showy; the chief are a Tillandsia, an Epidendrum and 
several Peperomieae. Of phanerogamic parasites there are four 
species of Phoradendron and two of Cuscuta. Flowers are generally 
small and inconspicuous, and in most cases regular and simple in 
structure. 
As regards its affinity the flora is an outlying portion of the 
American flora, with a strong specific differentiation, but it is impossi¬ 
ble to trace its relationship closely to any one section of the Pacific 
American vegetation. Nearly all the plants are identical with or 
obviously related to, species of the Sierras and Andes or of the 
Pacific slope between Lower California and Northern Chili. The 
xerophytic elements show a considerable resemblance to the desert 
flora of Southern Peru and the drier parts of the Andes, while the 
mesophytes correspond most nearly to plants of Ecuador, Colombia, 
Central America and Southern Mexico. 
Two views have been advanced regarding the origin of the 
islands. According to one view they are pelagic islands built up 
from the sea-floor by volcanic action; according to the other they 
are continental islands, tops of mountains formerly a part of the 
mainland, and now separated from it by subsidence. The first view is 
supported by the following facts :—(1) All parts of the islands now 
visible are obviously of volcanic origin. (2) The islands are 
separated from the mainland by a very considerable depth of ocean 
(more than 1,500 fathoms.) (3) The western coast of South 
America shows no signs of subsidence, but rather of marked elevation 
in geologic time. In accordance with this theory, the flora is 
assumed to have been brought by wind, oceanic currents and migra¬ 
tory birds. The islands lie in the course of fairly constant trade- 
winds, and the great ocean-current which sweeps along the west 
coast of Mexico, and the Humboldt current which runs northward 
along the coast of Chili and Peru, both turn westward just in the 
equatorial belt where the islands lie. Alexander Agassiz states 
that the velocity of the currents in the Panama district is very great, 
so that seeds, fruits, and masses of vegetation harboring small reptiles or 
even large ones, as well as other terrestrial animals need not be long 
afloat before they might be safely landed on the shores of the 
Galapagos; and as Hooker has shown, a large proportion of the 
plants are provided with special means of seed-dispersal. 
Against this view of the pelagic origin of the flora, Dr. Baur has 
adduced what he terms the harmonic character of the flora and 
fauna of the islands. Many plants and animals on the different 
islands are nearly related to each other without being exactly the 
same. F'or instance each of several islands has a peculiar species of 
Scalesia, one of the two endemic genera. There are also several 
different forms of Euphorbia viminea , several very nearly related 
