ABOVE: A dwarf , cultivated variety of Ficus microcarpa  known as Green Island with small round thick leaves  for sale in Kota Kinabalu. Many different varieties of Ficus microcarpa grown as bonsai  are popular throughout the world. F. microcarpa bonsai are especially popular in Taiwan where Ficus microcarpa grows naturally in the wild. These bonsai varieties  are often exported from Taiwan and sold in urban markets and garden centers  in Borneo under a large number of different names.

In Europe the most important commercial cultivation of bonsai is carried out in the Netherlands.

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This is a dwarf variety of Ficus microcarpa sold as a Ficus retusa bonsai. Ficus retusa is a completely different Ficus species. 
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This is a graft of two different varieties of F. microcarpa, one with enlarged roots and another with small leaves, in this case named Ficus ginseng  by the hopeful owner. However Ficus ginseng does not exist as a wild species of Ficus.
02 Microcarpa called Retusa in AVBJ (1918)
The mistake probably originated with this  illustration  of Ficus microcarpa  (labelled as Ficus retusa) growing in Java, copied  from  the Atlas der Baumarten von Java published in 1918 by Kooders and Valeton. The Dutch are not only famous as botanists in SE Asia, they are also some of the worlds most skilled horticulturalists. Thus when Dutch horticulturalists turned their skills to bonsai growing it was  an understandable error to name their Ficus microcarpa bonsai as Ficus retusa.
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Ficus retusa: To add to the confusion the Ficus species now called Ficus retusa illustrated above was called Ficus truncata in the Atlas der Baumarten von Java (1918).