824 GLOSSARY 



Glacial Period. — A period of great cold and of enormous 

 extension of ice upon the surface of the earth. It is 

 believed that glacial periods have occurred repeatedly 

 during the geological history of the earth, but the term 

 is generally applietl 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 alter- 

 ation 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 mem- 

 branes 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, character- 

 ized 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 



