main body, and islanded, as it were, on a remote
mountain-top in far warmer New Hampshire?

The answer is, they were stranded there at the end of the Glacial
epoch.

A couple of hundred thousand years ago or thereabouts--don't let us
haggle, I beg of you, over a few casual centuries--the whole of
northern Europe and America was covered from end to end, as everybody
knows, by a sheet of solid ice, like the one which Frithiof Nansen
crossed from sea to sea on his own account in Greenland. For many
thousand years, with occasional warmer spells, that vast ice-sheet
brooded, silent and grim, over the face of the two continents. Life was
extinct as far south as the latitude of New York and London. No plant
or animal survived the general freezing. Not a creature broke the
monotony of that endless glacial desert. At last, as the celestial
cycle came round in due season, fresh conditions supervened. Warmer
weather set in, and the ice began to melt. Then the plants and animals
of the sub-glacial district were pushed slowly northward by the warmth
after the retreating ice-cap. As time went on, the climate of the
plains got too hot to hold them. The summer was too much for the
glacial types to endure. They remained only on the highest mountain
peaks or close to the southern limit of eternal snow. In this way,
every isolated range in either continent has its own little colony of
arctic or glacial plants and animals, which still survive by
themselves, unaffected by intercourse with their unknown and
unsuspected fellow-creatures elsewhere.

Not only has the Glacial epoch left these organic traces of its
existence, however; in some parts of New Hampshire, where the glaciers
were unusually thick and deep, fragments of the primaeval ice itself
still remain on the spots where they were originally stranded. Among
the shady glens of the white mountains there occur here and there great
masses of ancient ice, the unmelted remnant of primaeval glaciers; and
one of these is so large that an artificial cave has been cleverly
excavated in it, as an attraction for tourists, by the canny Yankee
proprietor. Elsewhere the old ice-blocks are buried under the _debris_
of moraine-stuff and alluvium, and are only accidentally discovered by
the sinking of what are locally known as ice-wells. No existing
conditions can account for the formation of such solid rocks of ice at
such a depth in the soil. They are essentially glacier-like in origin
and chara