A species is precisely named by a two word combination—as Tyrannosaurus rex or Homo sapiens (modern man). These are Greek or Latin word combinations; the first word is capitalized and is the genus-name; the second is the species-level name. The binomial (e.g., genus and species) name of an organism must be written in italics. Paleontologists and biologists often change the higher levels of classification schemes for organisms as they improve the understanding of evolutionary relationships. The old division of life into
There are two classical definitions that separate animals from plants. The first makes the distinction by noting that the cell membrane of plants secretes a hard protective covering called a cell wall that is made of cellulose. An animal cell does not produce a cell wall. The second definition notes that plants manufacture their own food by photosynthesis. Animals must eat other organisms or the products of organisms. Animals can be divided into two major groups. The vertebrate animals (fish, amphibians, reptiles, birds, mammals [including humans]) have a backbone or similar stiffening rod and a dorsal nerve cord just above the backbone—these structures compose the spine in humans.
The following classification is used in throughout this document: