252 
PLANT MARCHES. 
Nov., 1890. 
Not so happy, however, was their return journey northwards 
when opportunity occurred. To say nothing of having to 
meet with new invaders from the north, they had become 
enervated through the genial influence of a more southern 
climate, and were now unequal to the ordeal of a return 
journey, consequently but a small proportion of the plant life 
of a former geological period found its way back to its old 
home. 
Another interesting result of these Plant Marches is to be 
found in the possible breaking up in the meantime of the 
previously existing land areas, which presented to the return¬ 
ing army impassable barriers of sea-girt lands, or snow-capped 
mountain chains; sometimes isolated areas, such as islands, 
or groups of islands, nourished and preserved for ages its 
stranded flora, or such of it as had been able to outlive inter¬ 
nal strife. These isolated spots are of extreme interest, 
whether with reference to fossilised, or existing, plants, as 
many ancient types have thus been more carefully and 
perfectly preserved. 
The disastrous effects of these insurmountable barriers 
upon the returning flora were the more telling, and the 
damage more apparent in Europe than in America. Over the 
latter continent it would appear—from Carboniferous times 
downwards—the floral Marches were more readily accom¬ 
plished than in the eastern hemisphere, consequently there 
lias always been on the American Continent an earlier appear¬ 
ance from the north of new types, and as less difficulties had 
to be encountered on return journeys, old types lingered 
longer, and eventually found their way back and re-established 
themselves further north than was the case in Europe and 
the east. 
The life of the Tulip Tree, already mentioned, is an 
instructive illustration of varied fortunes of plant migration, 
or, as I have termed it, the Plant March of its period. In 
Europe the Tulip Tree, like many of its American associates 
seems to have been destroyed by the cold of the Ice period, 
the Mediterranean cutting off its retreat. In America, however, 
it migrated southward over the southern extension of the 
continent, but returned far northward again with the 
amelioration of climate. 
The March and dispersion of plants through physical 
changes is strikingly illustrated in the scattering of the 
Northern Flora, driven southward at the time of the great 
Glacial period. Without commenting on the causes or duration 
of this later chapter of geological history, it will be sufficient to 
refer here to the general effect upon vegetation. To whatever 
