66 — Cincinnati Society of Natural History. 
the northeast coast of Asia.* With this connection the facilities for 
migration would be greater, and would be decidedly taken advantage 
of by animals and plants. There is also the same reason for supposing 
that, during that period, the stretch of ocean between Greenland, 
Iceland, and the northwest of Europe, was much less than it is now, 
even if the continents were not actually united. We know, further,t 
that at a former period of time, the climate there was such as to admit 
of the growth of plants, which are now strictly confined to temperate 
regions; and which certainly could not grow under the climatic con- 
‘ditions existing at the present day in the countries under the Arctic 
circle. | 
The resemblance between the floras of Europe and the United States, 
is by no means confined to the 360 identical species. There are, be- 
sides, many closely related species, some of which may be reduced to 
geographical varieties; and a still larger number of strictly represen - 
tative species, not likely to be confounded.{ If to the identical species 
we add these related and representative species, we shall find that one 
third of the 2,277 indigenous species given in Gray’s Manual, resemble 
European forms. But the similarity between the floras of North - 
America and Europe, is by no means confined to the small territory 
with which I have been dealing. I selected the portion ¢govered by 
Gray's Manual, because the country has been more thoroughly ex-. 
plored, and because the facilities for getting imformation are better 
than for other and larger tracts of country. It is well known that | 
many European forms extend along the Rocky Mountains to Colora- 
do, and other elevated localities, and I Have no doubt but that a com- 
parison of the entire flora of the United States (excluding the semi- 
tropical one of California, which really belongs to the Mexican region), 
will show nearly as much resemblance as I have shown exists in the 
small territory here dealt with. 
As for the glacial period and its effects upon the country, few are 
now inclined to deny it; and they grant not only the occurrence of 
cold periods but of warm ones intervening. Undoubtedly we may look 
to the north for the place of origin of many of our species of plants; 
. and in the glacial theory will find the principal factor for the disper- 
sion of the species “oo the place of their origination. 
' 
* A rise of less than 200 feet would ae this connection. Croll estimates that the fall of 
the level of the ocean in consequence of the withdrawal of water, was some 600 or 800 feet at 
Edinburgh, and would be more further toward the north. 
+ By the discovery of fossil plants at Disco, Greenland, and other places, and by the 
abundant coal fields of high northern latitudes. 
t For some of these species, see Am. Jour. Sci., /. ¢., vol. xxiii., pp. 80-85. 
