1 
E x 
78 Rhodora [APRIL 
is the probable climatic changes induced by the advance of the Pleisto- 
cene glaciers from the Arctic until they covered tbe larger part of 
North America, with the inevitable driving south of the flora and 
fauna to points whence it was possible for the boreal types to cross 
along the Cordilleras to South America. Finally, in the period of 
ameliorating climate accompanying or following the withdrawal of the 
glaciers, these plants, in order to maintain themselves in a suitable 
climatic zone, migrated towards either pole. This explanation seems 
satisfactory enough for the boreal genera and species, because a mere 
land connection between Patagonia and temperate North America is 
not enough; the climatie conditions prevailing over it needing to be 
such as to allow the arctic plants to migrate across. In order fully to 
justify this theory we should expect to find isolated on some of the 
higher mountains of Central America species which occur in temperate 
North America and Patagonia, or some that are found in North Amer- 
ica, but which did not succeed in making the journey to Patagonia. 
Such occurrences have been frequently demonstrated, as in the case 
of “the presence of Cystopteris fragilis and Phleum alpinum at an 
altitude of 12,000 feet on Mt. Orizaba in southern Mexico” ! and 
other similar cases. Both of the species cited occur in the cooler 
parts of North America and at the southern tip of South America. 
A more recent theory? is interesting in this connection: Scharff 
argues that this relationship between the two widely separated areas, 
North America and southern South America, is the result of a much 
more ancient dispersal including in the animal kingdom earthworms, 
slugs, salamanders, and mammals,’ and occurring “ probably during 
the dawn of the Tertiary Era." Geological charts illustrating the 
supposed land-areas during the Cretaceous and early Tertiary show a 
now obliterated land connection * between the western coast of North 
America at first with the southern tip of South America, and later 
with the northern part of the west coast of South America. Sharff’s 
most convincing argument for such a migration in Cretaceous or early 
Tertiary times is a quotation from Prof. Berry ? "that in mid-Creta- 
ceous times seventy-five percent of the known plants of Argentina 
were characteristic types of the Dakota-group flora of North America.” 
/ 
1 Fernald, Ruopona iv. 150 (1902). 
2 Scharff, Distrib. and Orig. of Life in Am. 413-419 (1912). 
3 Also ‘‘the genus Carabus, flightless beetles," Gadow, Wanderings of Animals, 90 
(1913). 
4 Scharff, 1. c. 280, 294, and Gadow, l. c. maps 11 and 12. 
5 Scharff, 1. c. 414. 
