would expect to see there. But although this reasoning helps
explain local differences in species diversity, it is less clear
whether history—like all the other explanations encountered so
far—can explain the global equator-to-pole gradient.  Glaciers
do not inevitably wipe out all species in their path. Rather
than being eliminated, the temperate zones and their species
might simply have shifted to follow the climate. This should be
particularly true in the sea, where life is more mobile. And the
climatic changes wrought by ice ages also affect tropical
forests. Fossil pollen shows that the extent of tropical forests
has varied a great deal. It appears that drier and warmer
climates lead to large patches of grassland appearing amid the
trees. It’s also been argued that isolating clumps of forest
from one another promoted diversity, because new species would
be more likely to evolve in forest fragments, just as they do on
islands. But you can’t have it both ways—either stability
promotes greater diversity or