It is necessary, however, to make a few observations on what is termed classification and its nature. Animals are classified by their resemblances and differences. Those creatures which resemble each other more than others are grouped together, and are separated from dissimilar groups. The first act in classification is to distinguish one animal from others by differences in the shape and internal construction, and the second is to group together the beings whose differences are small. A kind or species is a letter of the Zoological Alphabet, and it is usually said to refer to beings which produce others like unto themselves. A genus is a group of species closely resembling each other; a word in zoological language made up of few or many letters of the alphabet. There may be few or many species in a genus, and whilst some of them very closely resemble each other, others are not quite so much alike; and these link on one genus to another. The notion of a genus is to include a number of kinds in a group which has a character given to it: that is to say, certain peculiarities of shape and of anatomy. It will be obvious that the genus is an artificial affair, and is necessary for the purpose of making science easy. In order to explain this, look at a domestic cat, a lion, a tiger, a leopard, and a cheetah, and it will be observed that there are differences between them in shape and colour which cause them to be separated into distinct SPECIES. They all have some points of construction in common; and, therefore, they are classified together as five species of a GENUS--the genus _Felis_. Then consider the figure and colour of a hyæna, and of a civet, and study their internal anatomy, and it will be found that although there are differences between them which are sufficient to necessitate the placing of the hyænas in one genus (_Hyæna_), and the civets (_Viverra_) in another; yet the genera are closely united or allied, in consequence of their possessing many similarities. On comparing the genus _Felis_ with the genera _Hyæna_ and _Viverra_, it will be noticed that the last two resemble each other more than they do the first, and thus two FAMILIES are formed--one the _Felina_, to comprehend the genus _Felis_; and another the _Viverrina_, to include the genera of hyænas and the civets. But the slight resemblance between these families is sufficient to cause them to be grouped in an ORDER which is called Carnivora, or that of carnivorous beasts. Again, the Monkeys and Sloths do not resemble each other in shape and internal construction sufficiently to be placed in the same order even, but they and the Carnivora, and many other animals, suckle their young. They may, therefore, be separated, in a classification, from other animals which fly and lay eggs, and do not suckle: as the birds. The Birds form one CLASS, and the Mammalia, or animals that suckle their young, form another. Other Classes are formed by the Reptiles, Amphibia, Fishes, etc.