Today, scientists have discovered around 1.75 million species and new species are being discovered daily. From Aristotle's time to the late nineteenth century, the living things were classified as Plant Kingdom and Animal Kingdom. Later scientists probed into the simpler forms of life, which also needed to be classified.
History of Taxonomy
In the olden days, European and American scientists classified the organisms into various categories depending on their physical characteristics. Classification of living beings is otherwise known as "Taxonomy". The father of the modern taxonomy is Carolus Linnaeus. He was a Swedish botanical scientist and a medical doctor who lived in the 18th century.
Linnaeus has published around 180 books on plant species. "Systema Naturae", a book of his, which was published in 1735 outlined the scheme of classifying of living things. This book was the accepted by the fraternity of the scientists in the 19th century. The Linnean system of classification is the framework of the modern classification.
This system designates two Latin names to each type of organism. Living things were classified into three groups based on their genetic similarity. They are Archaea, Eubacteria and Eukaryota. The technical name for a group is domain. Each domain is classified into several groups called kingdoms. A kingdom was further classified on the basis of the species.
John Ray, an English naturalist, coined the term "Species". He made a contribution to modern taxonomy in book Historia Plantarum
Classification of Living Things
Living things are classified into five kingdoms. They are Prokarya, Animalia, Plantae, Fungi and Protoctista.
The livings things belonging to this kingdom have no membrane bound nucleus or the genetic mater of the cell. Hence are called "prokaryotes". An example of this kingdom is blue-green algae.
The living things that are classified under the kingdom "Animalia" are multi-celled organisms. They were developed as a result of the fertilization of an egg by a sperm. Certain species belonging to this kingdom reproduce asexually. For instance, a lizard belonging to genus Cnemidophorus.
The living things belonging to this kingdom are multi-celled organisms that reproduce through sexual fusion of a female and male cell. The plants fall under this kingdom.
The non-motile cells belong to the kingdom Fungi. The cell walls of these non-motile cells are made of chitin. There is still a debate on fungi. Some argue that a fungus belongs to the animal kingdom while there are some who think fungi should belong to plants. There is no embryonic stage for fungi. A fungus develops from spores. Fungi releases enzymes on the body of the other living things and thrive on them.
The living things that are not included in the above mentioned kingdoms are categorized here. Many of the microscopic organisms are included in this kingdom. The examples of this kingdom include protozoa, algae, slime molds, slime nets and so on.
The classical method of classification was through visual knowledge, and later through the microscopic study of the form of an organism. The classification reflects the evolutionary distances and relationships between various organisms.