genus includes Ilex aquifolium, which is native to Europe and known to
Americans as Christmas holly.) The trees in Ilex were like kids who spend
recess sprawled out on a bench. While Schefflera was sprinting upslope,
Ilex was just sitting there, more or less inert.
ANY species (or group of species) that can’t cope with some variation
in temperatures is not a species (or group) whose fate we need be
concerned about right now, because it no longer exists. Everywhere on
the surface of the earth temperatures fluctuate. They fluctuate from day
to night and from season to season. Even in the tropics, where the
difference between winter and summer is minimal, temperatures can vary
significantly between the rainy and the dry seasons. Organisms have
developed all sorts of ways of dealing with these variations. They
hibernate or estivate or migrate. They dissipate heat through panting or
conserve it by growing thicker coats of fur. Honeybees warm themselves
by contracting the muscles in their thorax. Wood storks cool off by
defecating on their own legs. (In very hot weather, wood storks may
excrete on their legs as often as once a minute.)
Over the lifetime of a species, on the order of a million years, longer-
term temperature changes—changes in climate—come into play. For the
last forty million years or so, the earth has been in a general cooling
phase. It’s not entirely clear why this is so, but one theory has it that the
uplift of the Himalayas exposed vast expanses of rock to chemical
weathering, and this in turn led to a drawdown of carbon dioxide from the
atmosphere. At the start of this long cooling phase, in the late Eocene, the
world was so warm there was almost no ice on the planet. By around
thirty-five million years ago, global temperatures had declined enough
that glaciers began to form on Antarctica. By three million years ago,
temperatures had dropped to the point that the Arctic, too, froze over,
and a permanent ice cap formed. Then, about two and a half million years
ago, at the start of the Pleistocene epoch, the world entered a period of
recurring glaciations. Huge ice sheets advanced across the Northern