Preformation.—The old conjecture (1672—Malpighi) that the development of an embryo was merely the expansion or unfolding of a miniature of the adult within the egg.
Protophyta.—Unicellular plants (q. v.).
Protoplasm.—Living matter.
Protozoa.—Unicellular animals (q. v.).
Representative Congenital characters.—See p. 65.
Reversion.—See Atavism.
Rudimentary Organs.—Usually considered a synonym of the term “vestigial characters,” and is the name under which are included all those organs which, either from having become useless or from other causes, have been much reduced in size, e.g. the muscles of the external ear in man (see Darwin and after Darwin, p. 76), &c. Latterly the former expression has been used to describe organs in process of development (e.g. the electric organ of the skate—loc.cit., p. 365 et seq.), whilst the latter is made to embrace all those organs in process of elimination.
Soma.—A general term descriptive of the whole mass of the body-cells of an organism.
Somatic-idio-plasm.—See p. 32.
Somatogenetic characters.—Characters acquired by the soma (i. e. variations acquired after birth by the action of the environment), as distinguished from characters produced and potentially present from the first by a union of two masses of germ-plasm—plasmogenetic characters (q. v.).