302 Willis .— The Origin of the THstichaceae and Podoste 7 naceae, 
walking about on the rocks afterwards with wet feet. But if the ancestral 
plants were water plants of quieter water, the seeds would never be in the 
least likely to come into contact with the wet feet of wading birds, for they 
would be shed into the water and not upon dry land. And they are in no 
way whatever suited to being shed into water. Even if it could be supposed 
that birds or other animals could in some way get them attached to them¬ 
selves, the chances of more than an occasional one or two arriving at any 
given rapid would be infinitesimal, and the chances of that one successfully 
germinating and attaching itself would be even less. 
Another fact that goes against the probability of water plants as 
ancestors is that the Tristichaceae and Podostemaceae could not, in their 
new mode of life, come into competition in any way with their ancestors, 
which would be living under entirely different circumstances. There does 
not, therefore, seem to be any reason whatever why the ancestral forms, or 
something fairly closely resembling them, should not still survive. Any 
catastrophe that so far upset the rivers as to destroy them would also 
probably destroy the Podostemaceae ; but in actual fact there are no other 
water plants living in quieter water anywhere that seem to show the very 
slightest relationship to these families. The only family at all nearly related 
to them among the water plants, the Hydrostachydaceae (formerly included 
in Podostemaceae), is also described as living in rapid water in the moun¬ 
tains, or in estuaries, 1 where the water is presumably also in movement 
to and fro. If there ever were any water plants of quiet water allied 
to these families, they have entirely disappeared. 
Then again, when we consider that the first adaptation of any plant, 
be it of land or of water, to live upon the rocks in rapid water, must have 
been by a single large mutation, and when we consider that a water plant of 
still water would in any case have to get rid of its large intercellular spaces, 
thus undergoing a mutation as large as that which a land plant would have 
to undergo, it is evident that, so to speak, we gain nothing by making 
the ancestor a water plant. 
It is thus fairly probable that these plants could not have arrived 
at the rapids where they commenced as seeds of other water plants living at 
a distance, when we consider the seeds which they actually possess, and 
which must represent the seeds of their ancestors. But there is still another 
possibility (2 a) open, that they may have crept into their habitats (or 
arrived as seeds) from the reaches between the rapids. These reaches, 
however, are absolutely without any other water plants, the only things that 
one finds in them being land plants which have crept into the water at the 
edges by means of runners, &c. They are floored with moving sand, upon 
1 This is a very interesting point; I have little doubt that some of the Podostemaceae would 
also survive under such conditions. Cf. also the Algae, described by Goebel, mentioned in my 
Indian monograph, Ann. Roy. Bot. Gard. Peradeniya, vol. i, p. 420. 
