OF CEYLON AND INDIA. 
451 
live in the lower levels of the country, in the larger streams, 
and in the more southern districts, Le., in the districts 
with the best distributed rainfall and the longest vegetative 
season. This may be best shown by a few examples. 
Thus in Ceylon the species that occur at the highest 
levels are Hydrobryum lichenoides and H. olivaceum^ 
while those that descend lowest are the large Dicræas. In 
the Nilgiris H. olivaceum is found at least as high as any, 
accompanied by the most prostrate of the Indian Dicræas, D, 
dichotoma, while the large species of the Malabar region, 
Tristicha ramosissima, Podostemon Barberi, Dicræa stylosa, 
&c., occur at low levels. Or again, to consider the size of the 
streams, whether in Ceylon, South India, the Bombay Ghats, 
or Assam, the only species found as yet in really small 
streams (often in mere becks of a yard or two in width) are 
Hydrobryums, Lawias, and Farmerias, with rare exceptions. 
Again, to deal with the latitudinal range, the species which 
go furthest into the more northern regions of short vegeta- 
tive season are also the Lawias and Hydrobryums, while the 
larger genera are confined to the more southern regions. 
Lastly, consider the immediate local habitat of the species 
growing at one place, e.g., at Hakinda. As has been fully 
described under the individual forms, each form has in 
general its own peculiarity of habitat, and is but little mixed 
with the others, except at places where the conditions vary 
very much in a small area, for instance where an eddy joins 
a swift current in the main stream. As will be easily seen by 
reference to the details of habitat given under each species, 
the larger forms live in the less rapid and violent water, 
which of course is also the less liable to rapid shallowing. 
It is thus evident that the dwarf forms affect, and are 
apparently best suited to, swift streams and rapid water which 
may easily or quickly become shallow. The question then 
arises, whether their adaptation is to the swiftness or to the 
shallowness, or to both. Probably, as has already been 
pointed out, the latter condition is the more important, as 
even the large forms can stand much swifter water than what 
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