plain
the manner in which such a vegetation lived through an arctic night of
several months' duration?[160]

It may seem premature to discuss this question until the true nature of
the fossil flora of the arctic regions has been more accurately
determined; yet, as the question has attracted some attention, let us
assume for a moment that the coal-plants of Melville Island are strictly
analogous to those of the strata of Northumberland--would such a fact
present an inexplicable enigma to the vegetable physiologist?

Plants, it is affirmed, cannot remain in darkness, even for a week,
without serious injury, unless in a torpid state; and if exposed to heat
and moisture they cannot remain torpid, but will grow, and must
therefore perish. If, then, in the latitude of Melville Island, 75
degrees N., a high temperature, and consequent humidity, prevailed at
that period when we know the arctic seas were filled with corals and
large multilocular shells, how could plants of tropical forms have
flourished? Is not the bright light of equatorial regions as
indispensable a condition of their well-being as the sultry heat of the
same countries? and how could they annually endure a night prolonged for
three months?[161]

Now, in reply to this objection, we must bear in mind, in the first
place, that, so far as experiments have been made, there is every reason
to conclude, that the range of intensity of light to which living plants
can accommodate themselves is far wider than that of heat. No palms or
tree ferns can live in our temperate latitudes without protection from
the cold; but when placed in hot-houses they grow luxuriantly, even
under a cloudy sky, and where much light is intercepted by the glass and
frame-work. At St. Petersburg, in lat. 60 degrees N., these plants have
been successfully cultivated in hot-houses, although there they must
exchange the perpetual equinox of their native regions, for days and
nights which are alternately protracted to nineteen hours and shortened
to five. How much farther towards the pole they might continue to live,
provided a due quantity of heat and moisture were supplied, has not yet
been determined; but St. Petersburg is probably not the utmost limit,
and we should expect that in lat. 65 degrees at least, where they would
never remain twenty-four hours without enjoying the sun's light, they
might still exist.

It should also be borne in mind, in regard to tree ferns, that they grow
in th