INTRODUCTION. 
xv 
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 
gioups. The fiist 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 a 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 cpiite 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 Falls. 
Then consider the figure and colour of a hyena, 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 hyenas in one genus (Hyama), 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 Fells with the genera Hyama 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 
Fehna, to comprehend the genus Fella; and another the Viverrina, to include the genera of hyenas 
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. 
All the animals of these numerous classes have a back-bone ; but if we examine a nautilus, a 
snail, a beetle, a worm, a coral, or an animalcule, nothing like an internal skeleton made up of bones, 
some of which are placed inside the back, can be discovered. Hence all the animals can be arranged 
into two sub-kingdoms, those with and those without back-bones, or the Vertebrata and the 
Invertebrata. (The name vertebrata is taken from the Latin word vertebra , which means a turning- 
joint in the body, or a back-bone.) Those are the sub-kingdoms of the animal kingdom, which is so 
called in contradistinction to the kingdom of plants. 
It must be remembered, however, that the best classification is but an attempt of a finite 
understanding to arrange the infinitely variable things of Nature. It is but an artificial and arbitrary 
arrangement which is necessary for study : for were the whole truth before us, there would be no 
