156 THE PLANT 
origin of polyphyletic species. In this connection, it hardly needs to be 
pointed out that this simple transfer of a species to several separated areas 
of a new habitat may often furnish complete proof that a new form may 
arise at different times, and at different places. 
194, Modification of the habitat. Efficient changes in the habitat are 
brought about by increasing or decreasing the water-content, or by varying 
the light intensity between sunshine and the diffuse light of deep forests. 
Humidity can not well be regulated except in so far as it is connected with 
water-content. Since its effects merge with those of the latter, its modifi- 
cation is unnecessary. An increase in water-content is readily brought about 
by irrigation. A stream may be dammed and its water allowed to spread 
over ane area to he studied, or the water may be carried to the proper place 
by deflecting the’ stream or 
by digging a canal. The 
construction of earth reser- 
oe makes it possible to 
‘dbtain almost any per cent 
of soil water by varying the 
' size of the, reservoir or the 
height of the wall or bank. 
: Near.a base station, such as 
Bays i fe ie Minnehaha, where there is a 
simple system of water- 
works, the experimental area 
_ may be watered whenever 
desirable by means of a hose. 
Water-content may be read- 
ily decreased by drainage, or 
by the deflection of a stream. 
When such means are not 
available, as in the case of 
extensive marshes, hum- 
mocks may be used or con- 
Fig. 47. Control ecad of Ranunculus sceleratus, structed, and the soil blocks 
poe ee containing plants placed up- 
on them. By the use of sand or gravel, the water-content of mesophytic 
areas can be reduced in a similar manner, or by surrounding the plant in situ 
with either of these soils which hold little water. In meadows, especially, 
the addition of a large quantity of alkaline salts decreases the amount of 
available water, while the holard may be reduced by denuding the soil about 
the plants concerned. 
