From the introduction:
"The genus Callosciurus Gray is an enormous taxon occurring over a great territory of the Indochinese and Malaysian subregions of the Oriental region. It is represenled by a large number of species and subspecies, many of which closely resemble each other and often show a considerable amount of colour variation, which makes the classification of this genus very difficult. Much work has been done on the taxonomic status, colour variation and distribution of its different species and subspecies. [...] most of the species have no definite pelage characteristics and there is great deal of variation among the dilferent subspecies of each species. They are very conservative in their cranial characters. Therefore, each species has to be defined by linking together populations that intergrade with each other, no matter how great the colour difference. In this paper an attempt has been made to make detailed taxonomic reassessment of some species and subspecies of the genus along with their geographical distribution."