arranged them into the system first suggested by Linnaeus. Whereas Linnaeus recognized about 9,000 species, systematists now have recognized about 1.5 million. The task of categorizing and describing species is still far from complete. Most species of smaller invertebrates, and many bacteria and other microscopic organisms, remain to be discovered. The plant kingdom is also incompletely known. Though the flowering plants of many areas, such as Europe and North America, are fairly well described, many other regions have not been nearly as well explored by botanists.
Recent investigations in the exceptionally diverse rainforests of South America have caused biologists to raise their estimates of the number of undescribed species. For example, a very high proportion of insects collected from the forest canopy are "new" species to science. It is now believed that the number of different species of plants and animals in the world may be ten million, or even more.
The scientific methods used in classifying organisms have been greatly improved over time. The process begins with the intensive field work in which the animals, plants, and microorganisms are collected and carefully examined. Most will be known to a specialist, but there might be some unusual examples. However, none is likely to be a complete stranger, since the specialist will probably recognize that any puzzling specimen is similar to some familiar species. Next the specialist must check all that has been published on the group of organisms that contains the similar species. If, after an exhaustive search, there is no record of a described species that corresponds to the one being examined, the specimen is probably a new species. The specialist will then prepare a careful description of the new species and publish it in a scientific journal. There is a permanent reward for being the describer of a new species: thereafter monographs that deal with the classification of the group to which the new species belongs will add the describer's name at the end of the scientific
name. Thus, for example, "Homo sapiens Linnaeus" is our own proper identification, because Linnaeus was the first to give us our scientific name.
This example makes it clear that not all scientific data are derived as the result of experimentation. The conventional classification of species into seemingly natural groups involved the careful observation of a variety of different species, followed by the use of selected characteristics in an attempt to define groups of species thought to be related. But the groupings are not always obvious. For example, it might have seemed reasonable to classify bats with birds, since the most conspicuous characteristic of each is the ability to fly. But bats are mammals. Like all mammals, their bodies are covered with hair and their young are born alive (instead of hatching from eggs) and are nourished by milk from the mother's mammary glands.
Although most of the species we know today were described after the time of Linnaeus, we continue to use his basic system of hierarchical classification. For example, similar genera are united in families, similar families in orders, similar orders in classes, and similar classes in phyla. The dog-like species listed above (the genus Canis ), plus a number of similar but more distant dog-like animals, are grouped as the family Canidae. This family plus the families of cats, bears, seals, and weasels form the order Carnivora. The carnivores and all other animals with hair are combined as the class Mammalia. Mammals are combined with the birds,