st snow closes the
passes. Here a zone of altitude, as in the polar regions a zone of
latitude, marks the limits of the habitable area.

[Sidenote: "Wallace's Line" a typical boundary zone.]

The distribution of animals and races shows the limit of their movements
or expansion. Any boundary defining the limits of such movements can not
from its nature be fixed, and hence can not be a line. It is always a
zone. Yet "Wallace's Line," dividing the Oriental from the Australian
zoological realm, and running through Macassar Strait southward between
Bali and Lombok, is a generally accepted dictum. The details of
Wallace's investigation, however, reveal the fact that this boundary is
not a line, but a zone of considerable and variable width, enclosing the
line on either side with a marginal belt of mixed character. Though
Celebes, lying to the east of Macassar Strait, is included in the
Australian realm, it has lost so large a proportion of Australian types
of animals, and contains so many Oriental types from the west, that
Wallace finds it almost impossible to decide on which side of the line
it belongs.[331] The Oriental admixture extends yet farther east over the
Moluccas and Timor. Birds of Javan or Oriental origin, to the extent of
thirty genera, have spread eastward well across Wallace's Line; some of
these stop short at Flores, and some reach even to Timor,[332] while
Australian cockatoos, in turn, have been seen on the west coast of Bali
but not in Java, Heilprin avoids the unscientific term line, because he
finds his zoological realms divided by "transition regions," which are
intermediate in animal types as they are in geographical location.[333]
Wallace notes a similar "debatable land" in the Rajputana Desert east of
the Indus, which is the border district between the Oriental and
Ethiopian realms.[334]

[Sidenote: Boundaries as limits of movements or expansion.]

Such boundaries mark the limits of that movement which is common to all
animate things. Every living form spreads until it meets natural
conditions in which it can no longer survive, or until it is checked by
the opposing expansion of some competing form. If there is a change
either in the life conditions or in the strength of the competing forms,
the boundary shifts. In the propitious climate of the Genial Period,
plants and animals lived nearer to the North Pole than at present; then
they fell back before the advance of the ice sheet. The restless
su