Abstract
There is a wide variety of animals, representing many different types of structures, from one-celled protozoa to the most complex animal man. There are animals with skeletons inside their bodies and some with skeletons on the outside. Some lay eggs and some bear live young; some are cold-blooded and some are warm-blooded. Within each group there are many species, each a little different from the others. For example, there are about 1200 species of rats and mice in the entire world. Some of those species are so similar that it is difficult to tell them apart, but each one is different in some way, and each species does not normally interbreed with any of the other species. Zoologists arrange these animals in a standard classification scheme, beginning with the simplest one-celled organisms and ending with man. In this classification system each type of organism is placed next to those that are most similar to it.