White Croats
It is considered that the Czech-Polish Croatian tribes were related to the Croatian tribes from Zakarpattia and Prykarpattia in Ukraine. They became separated during the migration period and seemingly formed one large Proto-Slavic tribe. It is theorized that the migration of those (White) Croatian tribes in the 7th century was the second and final Slavic migratory wave to the Balkans, which is related to the thesis by Bogo Grafenauer about the double migration of Slavs. According to this thesis, although it is possible that some Croatian tribes were present among Slavs in the first Slavic-Avar wave in the 6th century, it is argued that the Croatian migration in the second wave probably was not equally numerous to make a significant common-linguistical influence, while others considered they arrived in a significantly larger number. Zdenko Vinski and argued that the double migration thesis is supported by the rare findings of objects and ceramics of Prague-Korchak culture on the territory of Croatia, Bosnia and Herzegovina, and Serbia which were dated to the end of 6th and beginning of the 7th century (indicating first wave Slavs), and did not make a continuity with Slavic archaeological findings from the 8th-9th century, indicating assimilation. The Croatian tribes are also seen as a warrior group who assimilated into already present Slavs. In the territory of present-day Croatia they also gradually assimilated with the Pre-Slavic population because archaeology indicates a significant continuity of late antiquity population which have partially withdrawn to the mountains, coastal cities and islands.
Croatian literature
Croatian medieval prose was written in two languages: Croatian and Church Slavonic, using three different alphabets, Glagolitic, Latinic and Bosnian Cyrillic. Among these there was some interaction, as evidenced by documents carrying two forms of letters; especially with respect to Glagolitic and Cyrillic texts, and some Latin relied on Glagolitic forms. That interaction makes Croatian writing unique among Slavic prose, and even in European literature. Croatian medieval literature reflects the general trends within European literature, though there were some different traits, for example literature directed at the common people; a strong background tradition of oral literature, blending religious topics and interweaving of genres. A significant part of Croatian early literature is based on translations, with typical Central European edits. Croatian early literature was influenced from two spheres: from the East (Byzantine and Church Slavonic inheritance) and from the West (from Latin, Italian, Franco-Italian and Czech traditions).
From the 14th century, the western influence remained strong in Croatian literature. Recognizing these patterns, Croatian authors, mostly anonymous, adapted their work to the specific needs of the community in which and for which they wrote. Despite their writings being in large part translations, this literature achieved a notably artistic level of language and style. One of the most significant achievements was keeping alive the Church Slavonic literal language (especially in the Glagolitic alphabet). In later periods elements of that language came to be used in expressive ways and as a signal of "high style", incorporating current vernacular words and becoming capable of transferring knowledge on a wide range of subjects, from law and theology, chronicles and scientific texts, to works of literature. Such medieval works in the people's own language are the starting point for the literature of later periods. Anonymous poets and singers, developing their own styles, of typically religious poetry, and of this period, were referred to as the "začinjavci" by later authors and sources.