19th Century. Linguistics in the 19th century: historicism. Historical-comparative linguistics.
“…experience demonstrates that agreement in words is extremely uncertain. Through the intercourse of different peoples, an incredible number of words may pass from one language to another, however different the two may be in origin and type…. Grammatical agreement is a much more
It is one thing to see relations between sounds in two families (Latin qu- words (quis? who? etc.) correspond to Germanic hw (now English wh, German w);
But quite another to see that the sound changes are regular, and subject to sound law (regular sound change).
Spelling systems -- orthographies -- contain within the seed of a theory: that a language has just a small number of sounds that can organized (in a linear fashion…).
So while languages change, it is their categories of sounds that change. Languages don’t have residues of old sounds. Speakers at all times keep just a limited set of sounds.