RANGE. — The extent of country over which a plant or animal is naturally spread.

RANGE IN TIME expresses the distribution of a species or group through the fossiliferous beds of the earth’s crust.

RETINA. — The delicate inner coat of the eye, formed by nervous filaments spreading from the optic nerve, and serving for the perception of the impressions produced by light.

RETROGRESSION. — Backward development. When an animal, as it approaches maturity, becomes less perfectly organised than might be expected from its early stages and known relationships, it is said to undergo a RETROGRADE DEVELOPMENT or METAMORPHOSIS.

RHIZOPODS. — A class of lowly organised animals (Protozoa), having a