Continent world ultimately persisted with these being regarded as separate continents. From the late 18th century, some geographers started to regard North America and South America as two parts of the world, making five parts in total. Overall though, the fourfold division prevailed well into the 19th century. Europeans discovered Australia in 1606, but for some time it was taken as part of Asia. By the late 18th century, some geographers considered it a continent in its own right, making it the sixth (or fifth for those still taking America as a single continent). In 1813, Samuel Butler wrote of Australia
Smallest organisms 3.5 kilo base pairs (kb) and particle diameters of 80 nanometres (nm). The smallest double stranded DNA viruses are the hepadnaviruses such as Hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18-26 nm, but larger genomes, at 5 kb. It is important to consider other self replicating genetic elements, such as satelliviruses, viroids and ribozymes. The genome of "Nasuia deltocephalinicola", a symbiont of the European pest leafhopper, Macrosteles quadripunctulatus consists of a circular chromosome of 112,031 base pairs. "Nanoarchaeum equitans"'s genome is 490,885 nucleotides long. "Pelagibacter ubique" is one of the smallest known free-living bacterium