314 OKIGIN OF LIFE IN AMEKICA 



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. Kobinson 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. Kobinson and Greenman, J " 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. 

 t Ortmann, A. E., " Distribution of Decapods," p. 381. 

 I Eobinson, B. L., and J. M. Greenman, " Galapagos Flora," pp. 135 

 137. 



