It is believed that glacial periods have occurred repeatedly during the geological history of the earth, but the term is generally applied to the close of the Tertiary epoch, when nearly the whole of Europe was subjected to an arctic climate.
GLAND .-- An organ which secretes or separates some peculiar product from the blood or sap of animals or plants.
GLOTTIS .-- The opening of the windpipe into the oesophagus or gullet.
GNEISS .-- A rock approaching granite in composition, but more or less laminated, and really produced by the alteration of a sedimentary deposit after its consolidation.
GRALLATORES .-- The so-called wading-birds (storks, cranes, snipes, etc.), which are generally furnished with long legs, bare of feathers above the heel, and have no membranes between the toes.
GRANITE .-- A rock consisting essentially of crystals of felspar and mica in a mass of quartz.
HABITAT .-- The locality in which a plant or animal naturally lives.
HEMIPTERA .-- An order or sub-order of insects, characterised by the possession of a jointed beak or rostrum, and by having the fore-wings horny in the basal portion and membranous at the extremity, where they cross each other.

This group includes the various species of bugs.
HERMAPHRODITE .-- Possessing the organs of both sexes.
HOMOLOGY .-- That relation between parts which results from their development from corresponding embryonic parts, either in different animals, as in the case of the arm of man, the fore-leg of a quadruped, and the wing of a bird; or in the same individual, as in the case of the fore and hind legs in quadrupeds, and the segments or rings and their appendages of which the body of a worm, a centipede, etc., is composed.
The latter is called serial homology.

The parts which stand in such a relation to each other are said to be homologous, and one such part or organ is called the homologue of the other.

In different plants the parts of the flower are homologous, and in general these parts are regarded as homologous with leaves.
HOMOPTERA .-- An order or sub-order of insects having (like the Hemiptera) a jointed beak, but in which the fore-wings are either wholly membranous or wholly leathery, The Cicadae, frog-hoppers, and Aphides, are well-known examples.
HYBRID .-- The offspring of the union of two distinct species.
HYMENOPTERA .-- An order of insects possessing biting jaws and usually four membranous wings in which there are a few veins.

Bees and wasps are familiar examples of this group.
HYPERTROPHIED .-- Excessively developed.
ICHNEUMONIDAE .-- A family of hymenopterous insects, the members of which lay their eggs in the bodies or eggs of other insects.
IMAGO .-- The perfect (generally winged) reproductive state of an insect.
INDIGENES .-- The aboriginal animal or vegetable inhabitants of a country or region.
INFLORESCENCE .-- The mode of arrangement of the flowers of plants.
INFUSORIA .-- A class of microscopic animalcules, so called from their having originally been observed in infusions of vegetable matters.