Since the beginnings of the discipline, historical linguists have, like other scholars, used models to explore and explain the nature of their object of enquiry. By far the most popular of these models, in use since the nineteenth century, has been an evolutionary model based upon Darwinian notions of divergence between species.
Given the rapidity with which Charles Darwin's work came to dominate the intellectual paradigm of his time, it is not surprising that many nineteenth-century linguists were quick to adopt what they perceived to be Darwinian concepts and adapt them for the examination of linguistic evolution. Within a few years of the appearance of Darwin's The Origin of Species (1859), linguists such as Auguste Schleicher (Die Darwinische Theorie, 1863) and Max Müller (Lectures on the Science of Language, 1861-1864) had used Darwinian ideas for their own purposes; and by the end of the century evolutionary notions had become central to linguistic enquiry. Thus Hermann Paul, in the 'Neogrammarian bible' of late-nineteenth-century philological study Prinzipien der Sprachgeschichte (Principles of the History of Language, trans. H.A. Strong 1888), held that linguistic change was to do with 'the greater or lesser fitness of the forms which arise' (1888:13); in other words, the Darwinian-or perhaps, more correctly, sub-Darwinian-notion of 'the survival of the fittest' could be adapted for the use of historical linguists.
'The survival of the fittest', of course, is a phrase which has been widely reinterpreted by thinkers subsequent to Darwin. For Paul, the phrase meant survival of variant forms of highest frequency in a particular speech-community. Thus, he would hold that, when the p of Proto-Indo-European 'became' the f of Proto-Germanic, the former sound survived in some languages such as Italian (e.g. padre, pesce) but evolved in others (e.g. Present-Day English father, fish) because the Germanic community had determined (unconsciously, over many millions of usage-occurrences) that an f-type realisation of an earlier p was appropriate, and that other realisations were eccentric and thus insufficient for the clear expression of meaning.