Sentence:  the aspect of learning the internal organization of obvious natural genetic groups of languages as well as their more remote and elusive external links ; of <WSD>classification</WSD> first , with elapsed age merely a by-product ; of acquiring evidential knowledge of what happened in Athabascan , in Yokuts , in Uto-Aztecan in the last few thousand years as well as forecasting what more anciently may have happened between them . 
Meaning of the word marked with <WSD> tags:  the act of distributing things into classes or categories of the same type