_Scientific Arrangement._--Having thus cast a passing glance at the differences which exist between moths and butterflies, we take up the question of the subdivision of the butterflies into natural groups. Various systems of arranging butterflies have been suggested from time to time by learned writers, and for a knowledge of these systems the student may consult works which treat of them at length. It is sufficient for beginners, for whom this book is principally written, to observe that in modern science, for purposes of convenience, as well as from regard for essential truth, all individuals are looked upon as belonging to a _species_. A species includes all those individuals, which have a common ancestry, and are so related in form and structure as to be manifestly separable from all other similarly constituted assemblages of individuals. For instance, all the large cats having a tawny skin, and in the male a shaggy mane, constitute a species, which we call the lion; the eagles in the eastern United States, which in adult plumage have a snow-white head and neck and a white tail, constitute a species, which we know as the "white-headed" or "bald-headed" eagle. Species may then be grouped together, and those which are manifestly closely related to one another are regarded as forming a natural assemblage of species, to which we give the name of a _genus_. For example, all the large cats, such as the lion, the tiger, the puma, and the jaguar, are grouped together by naturalists, and form a genus to which is given the Latin name _Felis_, meaning _cat_. The name of the genus always comes before that of the species. Thus the tiger is spoken of scientifically as _Felis tigris_. The genera which are closely related to one another may again be assembled as _subfamilies_; and the subfamilies may be united to form _families_. For instance, all the various genera of cats form a family, which is known as the _Felidæ_, or the Cat Family. A group of families constitutes a _suborder_ or an _order_. The cats belong to the _Carnivora_, or order of flesh-eating animals. In zoölogy family names are formed with the termination _-idæ_, and subfamily names with the termination _-inæ_.