214 Glossary. 



Parthenogenesis. — A degenerate form of sexual reproduction, in which 

 the egg develops without having been fertilized by the male 

 element. 



Phylogeny. — The ancestral history of the race, as distinguished from 

 the life history of the individual (Ontogeny). 



Physiological Units (Spencer). — Special units which it is inferred 

 a plant or animal of any species is made up of, and in all of which 

 dwells the intrinsic aptitude to aggregate into the form of that 

 species. 



Plasma. — The constituent material of cells, e. g. germplasma (of sexual- 

 cells), somatoplasma (of body-cells). 



Plasmogenetic characters. — Variations due to admixtures of germ- 

 plasm in acts of sexual fertilization (and therefore present at birth), 

 as distinguished from somatogenetic characters — variations which 

 have been acquired independently of germ-plasm. See Somato- 

 genetic characters. 



Polar bodies. — Before an egg is fertilized the nucleus moves towards 

 the periphery and divides twice. The two cells that are thus 

 formed are the polar bodies. The extrusion of polar bodies is 

 probably universal among animals, but only one polar body is 

 extruded from parthenogenetic ova. See Darwin and after Darwin, 

 pp. 125 and 1 26. 



Preformation. — The old conjecture (1672 — Malpighi) that the de- 

 velopment 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 v 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 alter birth by the action of the environment), as 



