Sentence:  the aspect of learning the internal organization of obvious natural genetic groups of languages as well as their more remote and <WSD>elusive</WSD> external links ; of classification 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:  difficult to describe