_Palæolithic_: Belonging to the Old-Stone Age, which corresponds to the latter half of the Glacial or Pleistocene epoch. It is alleged to be the second period of prehistoric man (following the Eolithic) and is characterized by implements of unpolished stone shaped from flint by the chipping off of flakes of the latter substance. _Palæontology_: The science of fossil organisms. _Palæozoic_: A term applied to the second group of fossiliferous rocks, following the earliest, or Proterozoic, group, and preceding the Mesozoic group. It comprises the Cambrian, Ordovician, Devonian, Silurian, and Carboniferous systems, and its sediments are the first that contain well-preserved fossils. _Parasitism_: A condition in which one organism (the parasite) residing in, or upon, another species of organism (the host) lives at its expense, the relation being detrimental to the latter. _Parthenogenesis_: The production of offspring from unfertilized eggs. _Phenotype_: The sum-total of external characters by whose enumeration an organism is described—the somatic or expressed characters of an organism (or group of organisms) as distinguished from those that are merely potential in the germ cells. _Phylogeny_: Developmental history of the race, the hypothetical evolutionary history of the race, in contradistinction to the embryological development of the individual (ontogeny). _Phylum_: A term used in classification to denote any primary group of the plant or animal kingdom. _Plantigrade_: Walking on the whole sole of the foot, like bears. _Plastids_: Permanent organelles or living components of the cellular cytoplasm, _e.g._, chloroplasts, leucoplasts, etc. _Pleistocene_: The lower series of the Quaternary system of fossiliferous rocks. It corresponds to the so-called Glacial epoch, and extends from the close of the Tertiary period (system) to the dawn of the Recent or Historical epoch. _Polar Cell_: A synonym for polar body, or policyte. The polar bodies are minute abortive cells given off by the egg undergoing meiosis. Into them are shunted the chromosomes which the egg discards in its process of nuclear reduction (maturation). _Præformation_: Theory that the egg contains a complete miniature of the organism into which it develops. _Prehension_: Grasping, catching hold. _Progression_: Advancing movement, locomotion. _Pro-simiæ_: The lemurs as distinguished from genuine apes (Simiæ). _Protista_: Animals or plants which are normally unicellular and which when multicellular show no differentiation into tissues—the Protozoans and Protophytes. _Protoplasm_: Living matter. _Receptor_: An organ specialized to receive stimuli, _e.g._, a sense-organ.