314 
ORIGIN OF LIFE IN AMERICA 
which thus acted as a barrier in separating the North 
American from the South American fauna (see Fig. 17). 
That the distribution of reptiles and mollusks on the Gala- 
pagos islands points to a former land connection with the 
American mainland is especially urged by Dr. Arldt,* but, 
like Dr. Ortmann, he thinks the islands must have been iso¬ 
lated since the end of the Mesozoic era. The land connection 
referred to is shown in Dr. Ortmann’s f map illustrating the 
distribution of land and water during the Upper Cretaceous 
Period (Fig. 15). 
As regards the flora, Messrs. Robinson and Greenman con¬ 
tend that no species to be found on the different islands illus¬ 
trates the noteworthy racial divergence in related forms better 
than Euphorbia viminea. This species differs markedly in 
foliage from any other known member of the large genus 
Euphorbia and is characteristic of the Galapagos archipelago. 
Being essentially a desert plant, it can subsist even upon those 
islands of the archipelago which are of low altitude and do 
not attain the upper regions of moist atmosphere. The most 
cursory inspection of the forms from the different islands 
discloses marked variation in the contour, size, thickness, 
rigidity and colour of the leaves, as well as in the length of 
the internodes, colour of the stems and so forth, whereas more 
careful examination shows that these are not mere individual 
differences, due to chance, state of development, or individual 
environment, but that each form appears in general to be 
restricted to a single island. 
The question consequently presents itself, according to 
Messrs. Robinson and Greenman,$ “ If this archipelago is 
composed of islands of elevation, built up from the sea-floor 
independently by volcanic action, how has such a distribution 
been effected? ” If the vegetation has been derived from the 
mainland by chance transportation of seeds, it is quite impos¬ 
sible to believe that each island has received a slightly different 
form of the same species, and we are forced to the much more 
natural assumption that racial and varietal divergence has 
* Arldt, T., “ Entwicklung der Kontinente,” p. 116. 
f Ortmann, A. E., “ Distribution of Decapods,” p. 381. 
f Robinson, B. L., and J. M. Greenman, “ Galapagos Flora,” pp. 135 — 
137. 
