16 THE SUNKEN ISLAND OF ATLANTIS. 
Flora Tertiaria. Flora Boreali- Americana. 
P. balsamodes, U, P. balsameaj L, 
P. Leuce, U, P. alba. Ait, 
P, Grotheana, U. P. Teocote, Cham. 
P, ambigua, U, P. patula, Sehlecht, 
P. rigios, Z7. P. rigida, Mill, 
P. Metteniij U, P. Montsezumse, Lam, 
P. hepios, Z7, P. mitis, MieJix, , 
P. Freyeri, JJ. P- inops, Soland. 
P, centrotos, Z7, P. pungens, Michx. 
P. furcata, i7. P. Bantsiaua, Lamb, 
p. KotscLyana, 17. P, monticola, JDongl, 
P. spicseformis, Z7. P. Strobus, Z-. 
V 
That Europe was at one time covered witli plants, at present intro- 
duced into its gardens from a great distance ; that amongst them are 
not a few which even in America no longer exist or which do not stand 
a transplantation to our climate, — -is a strange phenomenon, demanding 
causes which must have wrought vast changes in the conditions under 
which these plants flourished, in the shape of the earth's surface and in 
the clhnate of these countries. The most singular is to find plants of a 
far distant continent strongly represented in the strata of our native 
country 3 whilst those of the adjacent eastern continent only appear in 
limited numbers. 
An explanation of this phenomenon we can hope to find only in the 
laws regulating the first appearance and distribution of plants. With- 
out entering too far into a subject by no means sufficiently cleared up, 
I may direct attention to some of the most striking laws at present 
admitted as regulating the geographical distribution of plants and 
equally in force in former geological periods. The plants of any given 
district can have taken possession of it in two ways only. They must 
either have been created in it — the species were formed on the spot 
or they reached it by some agency or other from abroad. In applying 
this to the lignite flora of Europe, we do not see any reason for as- 
suming that the plants which at that period existed in Europe were 
created in that continent. The striking resemblance which many of 
the most characteristic species bear to those nowadays living in North 
America, encourages us to suppose that some connection or other ex- 
isted between the two floras. Only one alternative is possible. Either 
our Molasse flora has gradually spread to North America, or the reverse 
