_Abiogenesis_: The discredited hypothesis that life may originate spontaneously in lifeless matter, _i.e._, apart from the influence of living matter. _Adaptation_: (1) The reciprocal aptitude of organism and environment for each other; (2) a structure, modification of structure, or behavioristic response enabling the organism to solve a special problem imposed by the environment; (3) the process by which the organism’s adjustment to the environment is brought about. _Allelomorphs_: Genes located opposite each other on homologous chromosomes and representing contrasting characters; they are separated during meiosis according to the Mendelian law of segregation, _e.g._ the genes for red and white in Four o’clocks which when united give rise to pink, and when segregated, to red and white flowers respectively, are allelomorphs of each other. _Alluvial_: Pertaining to the Alluvium, which consists of fresh-water deposits of the Pleistocene and Recent series, to be distinguished from the Diluvium which consists of older Pleistocene formations. _Amino-acids_: The chemical building-stones of the proteins—organic acids containing one or more amino-groups (—NH₂) in place of hydrogen, _e.g._, amino-acetic acid, CH₂·NH₂·COOH. _Amnion_: A membranous bag which encloses the embryo in higher vertebrates. The lower vertebrates, namely, fishes and amphibia, have no amnion and are termed “anamniotic.” The reptiles, birds, and mammals which possess it are termed amniotic vertebrates. _Amphioxus_: The most simply organized animal having a dorsal notochord. It is classified among the Acrania in contradistinction to the craniate Chordates which make up the bulk of the vertebrates. _Angiosperms_: The higher plants, which have their seeds enclosed in seed-vessels. _Anthropoid Apes_: Apes of the family SIMIIDÆ, which approach man most closely in their organization, namely, the chimpanzee, the gorilla, the gibbon, and orang-utan. _Antibody_: Chemical substances produced in the blood in reaction to the injection of antigens or toxic substances and capable of counteracting or neutralizing said substance. Such antibodies are specific for determinate antigens. _Antigen_: Any substance that causes the production of special antibodies in the blood of susceptible animals, after one or several injections. _Arthropods_: The phylum of exoskeletal invertebrates comprising crustaceans, arachnida, insects, etc. _Atavism_: The resemblance to an ancestor more distant than the parents. _Automatism_: A spontaneous action, not in response to recognizable stimuli. _Basichromatin_: That portion of a cell’s nuclear network which contains nuclein and is deeply stained by basic dyes. _Biparental_: Derived from two progenitors, _i.e._, a father and mother.