352 FEESII-WATER PRODUCTIONS. Ciixr. XII. 



fish not rarely dropped hy Avhirlwinds in India, and the vitality 

 t)f their ova when removed from the Avater. ]3ut I am inclined 

 to attribute the dispersal of frcsh-Avater iish mainly to changes 

 in the level of the land within the recent period having caused 

 rivers to flow into each other. Instances, also, coidd be given 

 of this having occniTcd during floods, without any change of 

 level. The wide dilTerence of the fish on the opposite sides 

 of continuous mountain-ranges, -which from an early period 

 must have completely prevented the inosculation of the river- 

 systems, seems to lead to the same conclusion. "With respect 

 to allied fresh-water fish occurring at very distant points of the 

 world, no doubt there arc many cases which cannot at present 

 be explained : but some fresh- Avater fish belong to very ancient 

 fonns, and in such cases there will have been ample time for 

 great geograpliical changes, and consequently time and means 

 for such migration. In the second place, salt-water fish can 

 Avith care be sloAvly accustomed to Ha'c in fresh Avater; and, 

 according to Valenciennes, there is hardly a single group of 

 Avhich all the members are confined exclusively to fresh Avatcr, 

 so that a marine species of a frcsh-Avater group might travel 

 far along the shores of the sea, and subsequently liecome modi- 

 fied and adapted to the fresh Avaters of a distant land. 



Some species of fresh-Avater shells haA'e A'crA' Avide ranges, 

 and allied species Avhich, on our theory, are descended from a 

 common parent, and must haA'C proceeded from a single source, 

 prevail throughout the Avorld. Their distribution at first per- 

 plexed me much, as their OA'a are not likely to be transported 

 by birds, and are immediately killed by sea-Avater, as are the 

 adults. I could not even understand hoAV some naturalized 

 species have spread rapidly throughout the same country. But 

 tAvo facts, Avhich I have observed — and no doubt many others 

 remain to be observed — throAV some light on this subject. 

 ^^'hen a duck suddenly emerges from a pond coA'cred Avith 

 duck-Avced, I have tAvicc seen these little plants adhering to 

 its back ; and it has happened to me, in remoAing a little 

 duck-AVoed from one aquarium to another, that I have quite un- 

 intentionally stocked the one Avith fresh-water shells from the 

 other. 13ut another agency is perhaps more eftectual : I sus- 

 pended a duck's feet in an aquarium, Avhere many ova of fresh- 

 Avater shells Avere hatching ; and I found that numbers of the 

 extremely minute and just-hatched shells craAvled on the feet, 

 and clung to them so firmly that, Avhen taken out of the Avater, 

 they could not be jarred off, though at a someA\hat more ad- 



