Li Wei (2000). The bilingualism reader, Psychology Press. "... linguistic distance is a notion which still remains problematic (for a discussion, see Hinskens, 1988), it does seem possible to place languages along a continuum based on formal characteristics such as the number of cognates in languages or sets of shared syntactic characteristics..."
Jones, A. A. (1998). Towards a Lexicogrammar of Mekeo (An Austronesian Language of West Central Papua), Volume 138 of Pacific Linguistics: Series C. Research School of Pacific and Asian Studies, Australian National University, Canberra.
Aristotle [1965-1991] History of animals, translated by Peck A.L. (Books 1-6) and Balme DM (Books 7-10). Loeb Classical Library, Harvard University Press, Cambridge, MA, 3 vols (The introductory quote to chapter 6 is from D'Arcy W. Thompson's translation,引咗嗰段英文係:"Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation..."
Irwin, D. E., Irwin, J. H., & Price, T. D. (2001). Ring species as bridges between microevolution and speciation. Microevolution rate, pattern, process, 223-243.
Gooskens, Charlotte (2007). "The Contribution of Linguistic Factors to the Intelligibility of Closely Related Languages". Journal of Multilingual and Multicultural Development. 28 (6): 445.
Gooskens, C., van Heuven, V. J., Golubović, J., Schüppert, A., Swarte, F., & Voigt, S. (2018). Mutual intelligibility between closely related languages in Europe. International Journal of Multilingualism, 15(2), 169-193.
Trudgill, Peter (2004). "Glocalisation and the Ausbau sociolinguistics of modern Europe". In Duszak, Anna; Okulska, Urszula (eds.). Speaking from the Margin: Global English from a European Perspective. Polish Studies in English Language and Literature 11. Peter Lang.
Alexander M. Schenker. (1993). "Proto-Slavonic," The Slavonic Languages. (Routledge). Pp. 60-121. Pg. 60: "[The] distinction between dialect and language being blurred, there can be no unanimity on this issue in all instances..."
C.F. Voegelin and F.M. Voegelin. (1977). Classification and Index of the World's Languages (Elsevier). Pg. 311, "In terms of immediate mutual intelligibility, the East Slavic zone is a single language."
Bernard Comrie. (1981). The Languages of the Soviet Union (Cambridge). Pg. 145–146: "The three East Slavonic languages are very close to one another, with very high rates of mutual intelligibility... The separation of Russian, Ukrainian, and Belorussian as distinct languages is relatively recent... Many Ukrainians in fact speak a mixture of Ukrainian and Russian, finding it difficult to keep the two languages apart..."