WATER-PLANTS 55 
multiply by vegetative means, and how difficult and 
uncertain seed-formation is with them. Again, the 
annual, represented in winter by seed only, is clearly 
more suited to life on land than in water, for it is here 
that winter conditions are more pronounced. Among 
the alleged aquatic annuals found in Great Britain may 
be mentioned the duckweeds (Lemna), Zannichellia, and 
awlwort (Subularia). 
The Origin of Aquatics. 
It is generally assumed that the first forms of life 
originated in water, and that the earliest plants on 
the globe were aquatics. By a gradual process of 
modification some forms became fitted to live on 
land, and these, emerging from the water, gradually 
established themselves on dry soil, and became the 
forerunners of all subsequent land-plants. That was a 
long time ago, when the world was still young, and the 
first stratified rocks were being laid down under water. 
But from that day to this, through the countless ages 
of geologic time, plants have been changing and modifying, 
the better equipped races ever driving the weaker ones 
out of the fair and pleasant places on the soil. The 
flowering seed-plant is a late arrival on the scene, the 
highest and the most successful expression of natural 
adaptation in the long line of descent of land-forms. 
Certainly no plant could have evolved the seed-habit 
while it was submerged in water ; the flower is a useful 
structure only in the air. From this we conclude that 
every seed-bearing plant that lives in the water to-day 
is not a primitive aquatic, but has been derived from 
ancestors that once lived on the land. In the struggle 
for existence, certain plants were bound to be driven 
off their habitats by stronger and better equipped 
competitors. Those among them that were able to 
adapt themselves to the conditions of other environ- 
ments, lived on; some took to the water, others to the 
hills, where competition was less keen. These were 
preserved from extinction; those which could not so 
adapt themselves perished. Our modern flowering 
aquatics therefore are, in every case, the descendants of 
plants which were thus worsted in the struggle for existence 
