andlocked dwindling relics
of ancient seas, and similar forms again would survive the change.

A typical assemblage of freshwater animals would include many Protozoa,
like Amoebae and the Bell-Animalcules, a representative of one family
of sponges (Spongillidae), the common Hydra, many unsegmented worms
(notably Planarians and Nematodes), many Annelids related to the
earthworms, many crustaceans, insects, and mites, many bivalves and
snails, various fishes, a newt or two, perhaps a little mud-turtle or in
warm countries a huge Crocodilian, various interesting birds like the
water-ouzel or dipper, and mammals like the water-vole and the
water-shrew.

Freshwater animals have to face certain difficulties, the greatest of
which are drought, frost, and being washed away in times of flood.
There is no more interesting study in the world than an inquiry into the
adaptations by which freshwater animals overcome the difficulties of the
situation. We cannot give more than a few illustrations.

(1) Drought is circumvented by the capacity that many freshwater animals
have of lying low and saying nothing. Thus the African mudfish may spend
half the year encased in the mud, and many minute crustaceans can
survive being dried up for years. (2) Escape from the danger of being
frozen hard in the pool is largely due to the almost unique property of
water that it expands as it approaches the freezing-point. Thus the
colder water rises to the surface and forms or adds to the protecting
blanket of ice. The warmer water remains unfrozen at the bottom, and the
animals live on. (3) The risk of being washed away, e.g. to the sea, is
lessened by all sorts of gripping, grappling, and anchoring structures,
and by shortening the juvenile stages when the risks are greatest.

V. THE DRY LAND

Over and over again in the history of animal life there have been
attempts to get out of the water on to terra firma, and many of these
have been successful, notably those made (1) by worms, (2) by
air-breathing Arthropods, and (3) by amphibians.

In thinking of the conquest of the dry land by animals, we must
recognise the indispensable role of plants in preparing the way. The dry
ground would have proved too inhospitable had not terrestrial plants
begun to establish themselves, affording food, shelter, and humidity.
There had to be plants before there could be earthworms, which feed on
decaying leaves and the like, but how soon was the debt repaid when