304 Willis .— The Origin of the Tristichaceae and Podostemaceae . 
mutation (or mutations) appeared which enabled them to live permanently 
in the water. The primary shoot, developed on land, is always a last 
resource for survival, whereas water plants arriving from other places— 
or land plants either, for that matter—have to come as seeds, few and 
far between, and have, so to speak, nothing to fall back upon. 
Having taken to the water like this, one can imagine. that these 
secondary shoots would flourish, for they would thus at one stroke acquire 
a large amount of virgin territory as yet free of any plants except an 
occasional Moss, and at the same time would come into a medium which 
afforded an abundant supply of food. The seeds produced upon them 
would still have a fair chance of reaching the bank in reasonable numbers. 
On the other hand, one can imagine the primary axis to some extent handi¬ 
capped by having to start the secondaries, and by the fact that their 
successful growth may interfere with its own, though neither of these causes 
is really likely to make much difference. But anyhow, it is not difficult to 
imagine it some day taking to the water itself, perhaps by the appearance 
of some mutation which might appear among the seedlings which would 
tend to arise upon the rocks from the seeds of the secondary shoots, or as 
a direct mutation, which would then become transmissible by the seeds 
which would be falling upon the rocks in any case. There is a difference in 
anatomy sometimes seen between primary and secondary shoots which may 
point to their having arisen at different periods in the phylogeny of the 
families. 
It is in some such way as this, we feel sure, that these families came 
into existence. Water or land plants from other places than close by could 
never have adapted themselves to this peculiar mode of life by the aid 
of seeds which would only reach the rocks on rare occasions, and would then 
have little or no chance of retaining their position. 
Be it noted that there was, so far as we can see, no absolute necessity 
for these ancestral plants to become Tristichaceae or Podostemaceae. They 
might quite well have remained amphibious. As mutation appears usually 
to be quite ‘ indefinite ’, giving rise to useless characters, and as the chance 
of a favourable mutation arriving among a chance possible assortment 
is almost infinitesimal, this would lead one to infer that here at least the 
appearance of the necessary final mutation was perhaps determined by the 
conditions of life, whether directly or indirectly. 
In any hypothesis whatever of the origin of these plants, a very large 
mutation is necessary, for the change of life in beginning to live as these 
families do is so great that no gradual change other than one something like 
that which we have sketched could effect it, and even in this the last 
transition must be ultimately effected by a sudden mutation. The great 
difficulty, upon any hypothesis, is to account for the appearance of this 
mutation, for we know that acquired characters, such as would be the habit 
