In the Domain Eukarya, there are four kingdoms. These four kingdoms define more specifically the characteristics of each of the organisms. Each kingdom has similarities to other kingdoms, however just like the domains, the kingdoms have differences that make each one separate. These kingdoms are the second most broad topics for the taxonomy. The kingdoms used to be the highest the most general classifying tool, however, once more research and more organisms were discovered, the domains became the broadest in the system.
Each kingdom has subtle differences, that makes the kingdom it's own kingdom. First off, they are all alike because they are eukaryotes, and they have the possibility to be unicellular and multicellular. The kingdoms Protista, Fungi and Plantae all do not have a nervous system like the organisms found in the kingdom Animalia. In addition, all the different kingdoms have the possibility to be asexual or sexual. However the organisms reproduce differently when they are reproducing asexually, including budding or spores. The kingdoms Fungi and Plantae are not motile like kingdoms Animalia and Protista. Kingdoms Protista, Plantae, and Animalia all have the possibility to consume nutrients becasue the organism is a heterotroph. Fungi is the only kingdom that contains organisms that are saprotrophs. Lastly, the kingdoms Protista and Plantae are the only kingdoms that have the option to use photosynthesis when obtaining nutrients.
How did it start?
The classification started with just the contrast between plants and animals. In the 18th century it then became animal, plant and mineral organism (Regnum Lapideum ). After electronic microscopes were created, it was discovered that there were also eukaryotes and prokaryotes. The prokaryotes were created into an additional kingdom called Monera. The position of Fungi was not created for a long time, but when it was, the organisms came from Plantae and Protista. In the 1970s, the prokaryotes were spilt into two kingdoms, Eubacteria and Archaea. This was the agreed upon classification throughout the 20th century. Now, however, the two prokaryote kingdoms have been made into Domains, and we now have the three domains. From the 21st century on, a phylogenetic approach to classify living beings has became the appropriate answer. After many tests and research, the real evolutionary relationship among Eukaryotes is still not clear. However there are groups who support the polygenic tree which includes the relationship between Plantae, red algae and green plants and algae. Other groups decided to also include certain Fungi and some amoeboid protozoa. There is even one group that connects the amoebozoa: amoebas and slime moulds. It is thought most of the organisms from the kingdoms have evolved from Protsita. But it is also clear that as a whole, they evolved from the Archaea and Bacteria. 
Figure 62: Cladogram of Kingdoms
Figure 63: Kingdom Evolution
Unity and Diversity
All the organisms that live are linked in some way, however what makes them unique is their diversity. The diversity was caused by time passing by and evolution taking its course. Even with two different organism looking nothing alike, they still have similar characteristics because all came from one organism that evolved over time.
Figure 58: Euglena gracilis
Figure 59: Golden snub-nosed monkey
The Euglena gracilis is in the kingdom Protista in the Domain Eukarya. Rhinopithecus roxellana is also known as the Golden snub-nosed monkey. This organism is part of the kingdom Animalia. It is a multicellular eukaryote. The Euglena gracilis is a unicellular eukaryote. They are both eukaryotes because both belong to same Domain. This means these two different organisms share the same organelles in its cell or cells. The Euglena gracilis is able to make its own food like the plants do. They are also able to eat organic matter like the animals. They eat green algae and this is what also gives them their color, which is similar to amoebas and paramecium.  The Golden snub-nosed monkey eats organic matter like the Euglena gracilis. Their diet depends on what the season is. During warm months they eat leaves from broad-leaf trees, pine needles, buds, bark, and fruit seeds. During the winter they eat bark and lichen. When they come to ground level, grass and wild onions are ingested too.  The Euglena gracilis and the Golden snub-nosed monkey obtain nutrition in the same way. However, the Golden snub-nosed monkey’s method is more complex in the process because the matter it obtains is more complex than the food of the Protist. The Euglena gracilis is motile by the flagella at the end with the movement like whips.  The Golden snub-nosed monkey is also motile. The process is more complex and consists of a nervous system. The nerves send for the arms/legs to move. This makes the organism able to swing from tree to tree or walk across the ground. Another thing it is able to do is communicate because of their brain and nervous system.  This is one of the characteristics that evolved from the Protist to the Animalia. The Euglena gracilis reproduces asexually via binary fission  because of its simplicity. The Golden snub-nosed monkey reproduces sexually because of its complexity. There is a mating season and the women indicate to them when want to reproduce.  This is another characteristic that has evolved over time.
Figure 64: Ginkgo biloba
Figure 65: Hydra viridissima
Click on one of the kingdoms below, to learn more about them!