Among the s and the water weeds of the shallows, lurk es, s, mud minnows, and young . All of these forage upon snails, crustaceans, and insect larvæ,… - Ann Haven Morgan

... The two problems which face every organism are those of maintaining its own life and continuing its race. Its youth is devoted entirely to satisfying its individual needs for food and safety; its adult life is devoted to the race, but the necessities of the individual are still satisfied though they may be secured in an entirely different way. The immature life of is aquatic, and to it all adjustments concerned with food or safety are exclusively confined. The mature or adult life is aerial. It is solely devoted to reproduction. There is no provision for food or for other means of lengthening its life. It gives an opportunity for studying ways of getting a living which have been completely isolated from ways of reproducing.

... 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.

… 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 s, the body fluids are practically filtered sea water.