214 The American Geologist. October, 1894 
of freshwater niollusks and fishes, these groups being taken 
for the reason that they record the clearest dependence on 
river history in their distribution. 
PART II. THE GENERAL EFFECT OF RIVER CHANGES ON 
FLUVIAL FAUNAS. 
It has been shown that fishes and niollusks are dispersed by 
a number of means, some of which, unless actually observed. 
offer to the investigator no evidence whereby they may be 
detected in after times. On the contrary, river changes, the 
topographic effects of which are still retained in the features 
of a country, afford, when studied in connection with the dis- 
tribution of its fishes and niollusks, a set of phenomena which 
it seems likely will at least in some instances prove instruct- 
ive. 
It has been customary to explain the phenomena of distri- 
bution in some of the following ways. Gunther,* noting that 
some freshwater fishes are able to migrate into salt water, 
supposes that the passage from one river to another may have 
been accomplished in this manner, or that there may have 
been a temporary passage opened across a watershed by a 
flood. Lyellf quotes Gmelin as authority for the statement 
that freshwater fish-spawn are distributed by anseres ; and 
he states further that scattered lakes are stocked by minute 
eggs, entangled in the feathers of the waterfowl, and that 
the water beetles (Dycticidae), which are amphibious and fly 
in the evening, may transport minute ova to distant pools, as 
when the fry of fish appear in rainpools. Darwin writes on 
this subject as follows: 
On the same continenl freshwater fish often range widely, and as if 
capriciously; for in two adjoining river systems some of the species 
may be the.same and seine wholly different. It is probable that they 
are occasionally transported by what may he called accidental means'. 
Thus fishes still alive are not very rarely dropped at distant points bj 
whirlwinds: and it is known that the ova retain their vitality for a con- 
siderable time after removal from the water. Their dispersal may, 
however, he mainly attributed to changes in the level of the land with- 
in the recent period, causing rivers to flow into each other. Instances 
also could be given of this having occurred during floods, without an\ 
change of level. The wide difference of the fish on the opposite sides of 
*An Introduction to the Stud} of Fishes. Edinburgh, 1880, pp. 211, 
212. 
f Principles of Geology, eleventh Am. ed., n. p. 374. 
