Acquired characters.—See Somatogenetic characters.

Amphigony (HÄckel).—Sexual reproduction.

Amphimixis (Weismann).—The mingling of the hereditary substances of two individuals in an act of sexual union.

Ancestral germ-plasm.—See p. 123.

Asexual Reproduction.—In which there is no liberation of special germ cells containing the potentiality of the adult organism, but in which the same object is effected by the liberation of buds, overgrowths, &c., which develop into the parent form. There are many forms of a-sexual reproduction.

Atavism.—The abnormal occurrence in existing species of characters which were peculiar to ancestral species, e.g., see Darwin and after Darwin, 2nd ed., Part I, p. 94.

Biophore.—See p. 123.

Blastogenetic characters.—See Plasmogenetic characters.

Calyx.—The outermost covering of the flower, which protects it before opening. Its position and precise function vary.

Cell nucleus.—A spherical or ovoid body embedded in the cell protoplasm, which has important functions in cell division and in reproduction. It consists of chromatin and achromatin. There are often several nuclei in one cell, whilst some cells have not been shown to have a nucleus at all.

Cessation of Selection (Romanes).—See Panmixia.

Chromatin threads.—Immediately before a cell divides the nucleus is resolved into chromatin fibres or threads and an achromatin matrix. These chromatin fibres are then marshalled into either rods or loops, &c., as the division of the cell proceeds (see Darwin and after Darwin, figs. 36, 37, and 38). Subsequent changes in the threads conclude the division (for a description of which consult the account above).

Chromosomes.—See Chromatin threads.

Compositae.—Plants in which the inflorescence consists of numerous small flowers brought together into a dense head, the base of which is enclosed by a common envelope (e.g. the Daisy, Dandelion, &c.).

Congenital characters.—See Plasmogenetic characters.

Conjugation.—This term is applied to a process observed in the Protozoa (q. v.), which seems to correspond to the sexual reproduction of the Metazoa (q. v.). The majority of the Protozoa cannot long continue to reproduce themselves a-sexually without becoming degenerate, or rather without becoming altogether extinct. Two individuals (as a rule) consequently unite either temporarily or permanently. In the former case, an exchange of material is effected; and in the latter, complete fusion takes place.

Correlation.—The normal coincidence of one phenomenon, character, &c., with another.

Cytoplasm.—See pp. 30 and 32.

Determinant.—See p. 123.

Ectoblast.—Syn. of epiblast and ectoderm. The general result of the division of a fertilized ovum is a two-layered ball of cells (a gastrula). The outer layer is called the ectoblast and the inner layer the entoblast. (See Darwin and after Darwin, p. 137 et seq.).

Embryology.—Hence embryogenesis, &c. The study of the development or the early growth of the individual.

Entoblast.—Syn. of hypoblast and endoderm. See Ectoblast.

Epigenesis (Harvey).—The theory that organisms are formed by the development of the egg itself, and not by the expansion of a miniature within the egg (preformation).

Fallopian Tubes.—The tubes through which the spermatozoa pass to effect fertilization, and through which the ova pass from the ovary to the uterus.

Fission.—Syn. of fissiparous separation. The breaking into two (without karyokinesis—q. v.) of a cell, which has, by overgrowth, disturbed its physiological equilibrium. This process is almost mechanical.

Formative material.—See p. 56.

Gemmation.—That form of a-sexual reproduction known as budding.

Gemmules (Darwin).—Minute granules, formed by the division of the general body-cells, which are supposed to be dispersed throughout the entire system. These themselves multiply by division, and are collected from all parts of the body to constitute the sexual elements.

Germ-plasm.—See p. 32.

Hydroids.—Belong to a division (Hydrozoa) of the stinging-animals or Coelenterata. They occur both in the sea and in fresh water, and are solely polypoid (i.e. tubular and tentacled).

Hydromedusae.—Also Hydrozoans. Hydroid colonies with special sexually reproductive persons, which are often liberated as floating bells or discs.

Idio-plasm (A and B).—See pp. 31 and 32.

Ids.—See p. 123.

Invertebrata.—Animals with a dorsal heart and without a backbone.

Karyokinesis.—The changes which are observed in the nucleus both immediately before and after cell division. See Chromatin threads.

Lamarckian factors. See Somatogenetic characters. Also Neo-Lamarckians.

Metaphyta.—Multicellular plants (q. v.).

Metazoa.—Multicellular animals (q. v.).

Micellae (NÄgeli).—See Molecules, with which they are identical.

Microaomata.—The protoplasm of certain vegetable cells is in places characterized by the presence of minute corpuscles, which may be regarded as part of the protoplasm, and are certainly of a protoplasmic nature. These are termed Microsomata.

Molecules (Weismann).—See p. 122.

Multicellular organisms.—Organisms composed of many cells, as distinguished from the Unicellular organisms, where each individual is constituted of only one cell.

Natural Selection.—Survival of the Fittest in the struggle for existence. For a full account of the process see Darwin and after Darwin, p. 251 et seq.

Neo-Darwinians.—Those who believe that Natural Selection has been the only modifying influence in the evolution of species, and that the material for its action has been only plasmogenetic characters (q. v.).

Neo-Lamarckians.—Those who hold that organic evolution has been effected solely by means of the occurrence and preservation (inheritance) of somatogenetic characters (q. v.).

Nuclear Thread or Loops.—See Chromatin threads.

Nucleo-plasm.—See pp. 30 and 32.

Nucleus.—See Cell nucleus.

Nutritive congenital characters.—See p. 64.

Ontogenetic grades.—See p. 35.

Ontogeny.—The life history of the individual, as distinguished from the ancestral history of the race (Phylogeny).

Ova.—Eggs—the product of the female reproductive gland (ovary or ovarium).

Ovule.—The seed in its earliest condition.

Pangenesis (Darwin).—The theory of Heredity by gemmules (q. v.).

Panmixia (Weismann).—The condition of free intercrossing, i.e. where Natural Selection (q. v.) cannot act.

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. germ-plasma (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 Somatogenetic 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 126.

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.).

Somatoplasma-plasm.—See p. 32.

Specialized congenital characters.—See p. 65.

Spermathecae.—Organs for the storing of the seminal fluid received in copulation.

Spermatogenesis.—The precise development of spermatozoa.

Spermatozoa.—The essential elements in the male seminal fluid, and secreted by the testis—the male reproductive gland.

Stirp (Gallon).—See p. 58.

Telegony.—See p. 141.

Unicellular Organisms.—Organisms composed of a single cell only, as distinguished from those consisting of aggregations of cells—Multicellular organisms.

Vertebrata.—Animals with a backbone and a ventral heart.

Vestigial Organs.—See Rudimentary organs,

Xenia.—See p. 141.