me of the most difficult to explain are presented by fish. It was
formerly believed that the same fresh-water species never existed on
two continents distant from each other. But Dr. Gunther has lately
shown that the Galaxias attenuatus inhabits Tasmania, New Zealand, the
Falkland Islands and the mainland of South America. This is a wonderful
case, and probably indicates dispersal from an Antarctic centre during a
former warm period. This case, however, is rendered in some degree less
surprising by the species of this genus having the power of crossing by
some unknown means considerable spaces of open ocean: thus there is
one species common to New Zealand and to the Auckland Islands, though
separated by a distance of about 230 miles. On the same continent
fresh-water fish often range widely, and as if capriciously; for in two
adjoining river systems some of the species may be the same and some
wholly different.

It is probable that they are occasionally transported by what may be
called accidental means. Thus fishes still alive are not very rarely
dropped at distant points by whirlwinds; and it is known that the ova
retain their vitality for a considerable time after removal from the
water. Their dispersal may, however, be mainly attributed to changes in
the level of the land within the recent period, causing rivers to flow
into each other. Instances, also, could be given of this having occurred
during floods, without any change of level. The wide differences of the
fish on the opposite sides of most mountain-ranges, which are continuous
and consequently must, from an early period, have completely prevented
the inosculation of the river systems on the two sides, leads to the
same conclusion. Some fresh-water fish belong to very ancient forms,
and in such cases there will have been ample time for great geographical
changes, and consequently time and means for much migration. Moreover,
Dr. Gunther has recently been led by several considerations to infer
that with fishes the same forms have a long endurance. Salt-water
fish can with care be slowly accustomed to live in fresh water; and,
according to Valenciennes, there is hardly a single group of which
all the members are confined to fresh water, so that a marine species
belonging to a fresh-water group might travel far along the shores
of the sea, and could, it is probable, become adapted without much
difficulty to the fresh waters of a distant land.

Some species of fre