For instance; in considering classes of animals, we abstract the combined quality of milk-giving and pouch-possessing which is possessed in common by a number of animals; then we group these several animals in a class which we name the _Marsupialia_, of which the opossum and kangaroo are members. In these animals the young are brought forth in an imperfect condition, undeveloped in size and condition, and are then kept in the pouch and nourished until they are able to care for themselves. Likewise, we may abstract the idea of the _placenta_, the appendage which connects the young unborn animal with the mother, and by means of which the foetus is nourished. The animals distinguished by this quality are grouped together as the Placental Mammals. The Placental Mammals are divided into various groups, by an Abstraction of qualities or class resemblance or difference, as follows: The _Edentata_, or toothless creatures, such as the sloths, ant-eaters, armadillos, etc.; the _Sirenia_, so-named from their fancied resemblance to the fabled "sirens," among which class are the sea-cows, manatees, dugongs, etc.; the _Cetacea_, or whale family, which although fish-like in appearance, are really mammals, giving birth to living young which they nourish with breast-milk, among which are the whales, porpoises, dolphins, etc.; the _Ungulata_, or hoofed animals, such as the horse, the tapir, the rhinoceros, the swine, the hippopotamus, the camel, the deer, the sheep, the cow, etc.; the _Hyracoidea_, having teeth resembling both the hoofed animals and the gnawing animals, of which the coney or rock-rabbit is the principal example; the _Proboscidea_, or trunked animals, which family is represented by the various families of elephants; the _Carnivora_, or flesh-eaters, represented by various sub-families and species; the _Rodentia_, or gnawers; the _Insectivora_, or insect feeders; the _Cheiroptera_, or finger-winged; the _Lemuroidea_, or lemurs, having the general appearance of the monkey, but also the long bushy tail of the fox; the _Primates_, including the monkeys, baboons, man-apes, gibbons, gorillas, chimpanzees, orang-outangs and Man. In all of these cases you will see that each class or general family possesses a certain _common quality_ which gives it its classification, and which quality is the subject of the Abstraction in considering the particular group of animals. Further and closer Abstraction divides these classes into sub-classes; for instance, the family or class of the _Carnivora_, or flesh-eaters, may be divided by further Abstraction into the classes of seals, bears, weasels, wolves, dogs, lions, tigers, leopards, etc. In this process, we must first make the more general Abstraction of the wolf and similar animals into the dog-family; and the lion, tiger and similar forms into the cat-family.