… Wherever animals live, in fresh water, salt water, or on land, their body fluids are similar; all are salty. In marine invertebrates, whether es or… - Ann Haven Morgan

... Darwin ... became the official naturalist on the five-year voyage of the "" (1831–1836) ...
... He also read the essay in which Malthus told of the human populations that became too large for the space available to them (1838). This suggested a plan. Some organism must be winnowed out by their natural surroundings; thus, others would be benefited. There would be natural selection.

Different waters hold their own special communities; the dainty glen stream shelters companies of mayflies in its swift riffles; pond shallows and meadow brooks are the homes of lurking ; and wayside puddles are populous with mosquito wrigglers and . In all these places living things must contend with winter cold and summer drought, with storms and flood waters. In winter the pond populations drop to the bottom, frogs and turtles dig under mud and broken plants, whirligig beetles hide under banks to come out with every warm spell, and fresh water sponges are packed in tough covered capsules. In summer when its own pool dries up the water boatman flies to some other pond but many caddis worms burrow into the mud bottom and endure the drought as best they can.

... s feed upon microorganism from the muck and water of the bottom and in turn are consumed by es, s and mankind.