ABSTRACT
One of the things which zoologists have to do is to classify animals. Every animal is assigned to a species. Similar genera are put in the same family, similar families in the same order, and similar orders in the same class and similar classes in the same phylum. The rodents are included in the class Mammalia, hairy warm-blooded animals which suckle their young, The Mammalia are one class of the phylum Vertebrata, which agree in having backbones, and in many other characters. There are also intermediate divisions, such as suborders and subclasses. So the class of Reptiles is a group of animals which have stayed at a certain level of organisation, rather than all the living descendants of a common ancestor. It is probable that animals originated from plants on a great many occasions, generally by a sudden leap of this kind, and it is likely that the first animals were flagellates.
