[…] cessation of communication, of a common regulative influence over the never-ending changes of speech, may seem a very slight cause of divergence, and so in truth it is; but it is fully sufficient to account for all the phenomena of dialectic growth. No matter how small the angle may be between two lines starting from the same point; if they are protracted far enough, their extremities may be found any given distance apart. And the angle of dialectic divergence is practically an increasing one […].
- Whitney (1875), Pag. 165

[…] so long as every change which arises in the local parts A and B and C, and so on, works its way through all the rest […] so long will the language X remain one […] But separate […] the parts A and B and C from one another, so that the changes in each are made in that alone, and do not extend into the rest, and the peculiarities of each will begin to be confined to itself; what we call dialectic growth will set in; the process of divarication into diverse languages will have begun. A brick wall, high enough and long enough, between the sections, would perfectly accomplish their division, and initiate dialectic divergence; only, of course if the separation takes place by local removal, so that the sections are brought into different external circumstances of nature and occupation, and under different historical influences, the process of linguistic divergence will be quickened.
- Whitney (1875), Pag. 164