Origin of Species Variorum

↑ 4 blocks not present in 1872; present in 1859 1860 1861 1866 1869

In regard to fish, I believe that the same species never occur in the fresh waters of distant continents. But on the same continent the species often range widely and almost capriciously; for two river-systems will have some fish in common and some different. A few facts seem to favour the possibility of their occasional transport by accidental means; like that of the live fish not rarely dropped by whirlwinds in India, and the vitality of their ova when removed from the water. But I am inclined to attribute the dispersal of fresh-water fish mainly to slight changes within the recent period in the level of the land, having caused rivers to flow into each other.

↑ 1 blocks not present in 1869 1872; present in 1859 1860 1861 1866

We have evidence in the loess of the Rhine of considerable changes of level in the land within a very recent geological period, and when the surface was peopled by existing land and fresh-water shells.

→ are continuous, and which consequently must from 1872

from 1859 1860 1861 1866 1869

→ OMIT 1869 1872

parted river-systems and 1859 1860 1861 1866

→ of the river-systems on the two sides, 1872

seems to 1859 1860 1861 1866

of the river-systems, seems to 1869

→ Some 1872

With respect to allied 1859 1860 1861 1866 1869

→ OMIT 1872

occurring at very distant points of the world, no doubt there are many cases which cannot at present be explained: but some fresh-water fish 1859 1860 1861 1866

occurring at very distant points of the world, no doubt there, are many cases which cannot at present be explained: but some fresh-water fish 1869

→ OMIT 1872

In the second place, 1859 1860 1861 1866 1869

→ which all the members are confined 1872

fishes confined exclusively 1859 1860 1861 1866

which all the members are confined exclusively 1869

→ OMIT 1869 1872

we may imagine that 1859 1860 1861 1866

→ species belonging to 1872

member of 1859 1860 1861 1866

species of 1869

→ could, it is probable, become adapted without much difficulty 1872

subse- quently become modified and adapted 1859 1860

subsequently become modified and adapted 1861 1866 1869

→ ova, as well as the adults, are 1872

are 1859 1860 1861 1866 1869

→ sea-water. 1872

sea water, as 1859

sea-water, 1860 1861 1866 1869

→ no doubt will 1872

remain to 1859 1860 1861 1866 1869

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 difference of the fish on

opposite

the opposite

sides of

continuous

most

mountain-ranges, which →are continuous, and which consequently must from an early period

must

....

have →OMIT completely prevented

their

the

inosculation,

inosculation

→of the river-systems on the two sides,

lead

leads

this

the

same conclusion. →Some fresh-water fish →OMIT 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. Günther has recently been led by several considerations to infer that with fishes the same forms have a long endurance. →OMIT

salt-water

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 →OMIT 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 fresh-water shells have

....

very wide

range,

ranges,

and allied

species,

species

which, on

our

theory, are descended from a common

parent

parent,

and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me much, as their ova are not likely to be transported by

birds,

birds;

and

they

~~the~~

the