This is one of the most common means of definition. The entity or word being defined (called the is first set into its genus (class) and then distinguished from other members of that class:
History is the recital of facts given as true, in contradistinction to the fable, which is the recital of facts given as false. Voltaire
Voltaire begins by setting "history" (the thing, not the word) into the genus "recital of facts." Then he differentiates it from the other member of that class, "fable."
The bulk of a genus-species definition usually goes to differentiation. This may be done explicitly, as in Voltaire's case; that is, you actually mention the other member(s) of the class and explain how the definiendum differs from them. Or it may be done implicitly, where you do not actually name the other member(s) of the class but simply describe the defini-endum so completely that it is, by implication, differentiated from them. Obviously a class of any size makes complete explicit differentiation impractical. If you were defining, say, football, it would take many, many pages to distinguish it from every other team sport.
However you differentiate the thing you are defining, you must be clear about which of its attributes are essential and which are not. For example, the fact that football is played in ways. For convenience a time. However, they practice they are often stadiums (usually outdoors) before large crowds is not essential to its definition: baseball and soccer are also team sports played under similar conditions. On the other hand, the rules of football, the dimensions and the markings of the these facts are unique. Such essential attributes are what distinguish a definiendum. But this does not mean that you should ignore incidental attributes altogether. If you were explaining football to a foreign friend, it would be important that he or she understand something about where and when it is played.
The following explanation of what a map is illustrates a genus-species
A map is a conventional picture of an area of land, sea, or sky. Perhaps the maps most widely used are the road maps given away by the oil companies. They show the cultural features such as states, towns, parks, and roads, especially paved roads. They show also natural features, such as rivers and lakes, and sometimes mountains. As simple maps, most automobile drivers have on various occasions used sketches drawn by service station men, or by friends, to show the best automobile route from one town to another.
The distinction usually made between "maps" and "charts" is that a chart is a representation of an area consisting chiefly of water; a map represents an area that is predominantly land. It is easy to see how this distinction arose in the days when there was no navigation over land, but a truer distinction is that charts are specially designed for use in navigation, whether at sea orfn the air.
Maps have been used since the earliest civilizations, and explorers find that they are used in rather simple civilizations at the present time by people who are accustomed to traveling. For example, Arctic explorers have obtained considerable help from maps of the coast lines showing settlements, drawn by Eskimo people. Occasionally maps show not only the roads, but pictures of other features. One of the earliest such maps dates from about 1400 B.C. It shows not only roads, but also lakes with fish, and a canal with crocodiles and a bridge over the canal. This is somewhat similar to the modern maps of a state which show for each large town some feature of interest or the chief products of that town. c. C. Wylie
Wylie first places "map" in its genus ("a conventional picture of an area of land, sea, or sky") and illustrates it ("road maps"). Next he distinguishes "map" from the other member of its class ("chart"). Finally, in the third paragraph, he gives us information about maps which, although not essential to the is interesting and enlightening.
In working out a genus-species definition, then, the essential questions to ask yourself are these:
To what class does it belong?
What unique qualities distinguish it from other members of that class?
What other qualities—even though not unique—are important if readers are fully to understand the word or thing?
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