Charles Darwin's 'Origin of Species' and the Scholars from Leipzig.

In the mid-19th century, Darwin published his famous Origin of Species, putting forward the theory of evolution. It seemed natural to attempt to chart the evolution of language alongside the evolution of species.

This emphasis on language change eventually led to a major theorical advance. In the last quarter of the century, a group of scholars centred around Leipzig, and nicknamed the 'Young Grammarians', claimed that language change is 'regular'. They argued that if, in any of a given dialect, one sound changes into another, the change will also affect all other occurrences of the same sound in similar phonetic surroundings.
The influence of the 19th-century scholars was strong. Even today, one still meets members of the general  public who expect the cataloguing of linguistic changes and the reconstruction of Proto-Indo-European to be the central concern of modern linguistics.