with its height and density. If there is no atmosphere
there can be no pumping; or if there is an atmosphere of less pressure
than ours, water such as ours can only be pumped to a less height than
33 feet. Again, we know that there are arctic regions in Mars; if there
are also arctic animals, are they white? That may depend upon whether
there are any beasts of prey. If not, concealment seems to be of no use.

An Empirical Law, being one whose conditions we do not know, the extent
of its prevalence is still less ascertainable. Where it has not been
actually observed to be true, we cannot trust it unless the
circumstances, on the whole, resemble so closely those amongst which it
has been observed, that the unknown causes, whatever they may be, are
likely to prevail there. And, even then, we cannot have much confidence
in it; for there may be unknown circumstances which entirely frustrate
the effect. The first naturalist who travelled (say) from Singapore
eastward by Sumatra and Java, or Borneo, and found the mammalia there
similar to those of Asia, may naturally have expected the same thing in
Celebes and Papua; but, if so, he was entirely disappointed; for in
Papua the mammalia are marsupials like those of Australia. Thus his
empirical law, 'The mammalia of the Eastern Archipelago are Asiatic,'
would have failed for no apparent reason. According to Mr. Wallace,
there is a reason for it, though such as could only be discovered by
extensive researches; namely, that the sea is deep between Borneo and
Celebes, so that they must have been separated for many ages; whereas it
is shallow from Borneo westward to Asia, and also southward from Celebes
to Australia; so that these regions, respectively, may have been
recently united: and the true law is that similar mammalia belong to
those tracts which at comparatively recent dates have formed parts of
the same continents (unless they are the remains of a former much wider
distribution).

A considerable lapse of time may make an empirical law no longer
trustworthy; for the forces from whose combination it resulted may have
ceased to operate, or to operate in the same combination; and since we
do not know what those forces were, even the knowledge that great
changes have taken place in the meantime cannot enable us, after an
interval, to judge whether or not the law still holds true. New stars
shine in the sky and go out; species of plants and animals become
extinct; diseases die ou