Baseleveling and Organic Evolution. — Woodworth. 217 
case the same species have a more extended range towards the 
sea to which the salt water is withdrawn. 
As a consequence of uplift of the land and the revival of 
streams which have attained their baselevel of erosion, rapids 
and falls may be introduced in their beds, Claciation may 
induce the same effect through the irregular deposition of 
glacial drift. Some fishes before capable of occupying the 
greater portion of a river basin may thus be limited in range. 
Essentially marine organisms, capable of enduring temporarily 
fresh water, may be excluded from ranging above the falls. 
Anadromous fishes of the families Clupeidae and Salmonidae, 
including the shad and salmon, have the habit of ascending 
large rivers into fresh water to breed. Shad ascend the riv- 
ers in the spring, their ascent being limited by unsurmount- 
able falls and unfavorable temperature. It is probable that 
this habit of leaping falls has been acquired, and it points to 
a time when the practice of ascending to the headwaters of 
rivers did not require the exercise of this function. These 
fish seem then to afford us an instance in which a geographic 
change, common to the rivers of the northern, glaciated 
lands, has been met by the acquirement of a habit of leaping 
falls, introduced into the streams by recent uplift or glacial 
derangement. Without this habit, the uplift of the land, or 
the formation of an axis of warping across the path of a 
stream, or even differential wear on the bed rock so as to form 
a fall, might have proved destructive to a species. Instances 
of this kind incline us to be cautious in speculating on the 
extent to which geographical changes of ;i minor kind exercise 
a life and death influence on particular species of animals. 
Effect ok Baseleveling ok a Mountainoi s Region. 
Mr. Alfred K. Wallace has recently considered the effect of 
geographic vicissitudes on organisms, under the title of 
"Changed Conditions."* After enumerating the ordinary 
modifications of land and water and dependent climate in 
their action upon vegetation, and so on herbivorous animals, 
lie notes that : "When such physical changes as these have 
taken place, it is evident that many species must either be- 
come modified or cease to exist."" He further draws the con- 
*l);ii-\voiisni. chapter v. 
