No. 418.] BASELEVELING. 841 
occupy the habitat that is increasing in area. These changes 
wil produce tension lines between the inhabitants of the 
various habitats. 
With regard to the river fauna, at first when the fall of the 
river is great, rapid-water forms have the advantage, but with 
the development of sluggish, meandering streams upon bottom 
land, quiet-water forms find a home. Since the lowland or 
swampy conditions begin at the mouth of the stream, quiet- 
Fic. 1.— Young valleys. (After Salisbury.) 
Water forms will be introduced there, and with the noe 
"p stream of lowland conditions this fauna is led in inn — 
tion ; similarly the head of the valley leads its rapid-water 
fauna in the same direction. Thus there is a definite succes- 
"lon of forms working up the valley, and, in a oe 
by passing down stream one gets a recapitulation ien gun 
of fauna which inhabit a given point of the stream during Its 
Ideal history, Dendritic tributary streams help to carry vois 
same conditions in diverging directions, thus leading the fauna 
n Various directions toward the divides. 
1 
