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 cetacean, which is also (as of 2006) the most endangered, is the vaquita. Male vaquitas grow to an average of around 135 cm (53 in); the females are slightly longer, averaging about 141 cm (55 in) in length. Duckweeds of the genus "Wolffia" are the world's smallest flowering plants. Fully grown, they measure only 300 µm by 600 µm and reach a mass of just 150 µg. Nanobes are thought by some scientists to be the smallest known organisms about one tenth the size of the smallest known bacteria. Nanobes, tiny filamental structures first found in some rocks and sediments,