There are […] two fundamental rules, under the government of which all comparative processes must be carried on : 1. comparisons must have in view the established lines of genetic connection, and 2. the comparer must be thoroughly and equally versed in the materials of both sides of the comparison […] if they be duly heeded, there is no limit to the scale on which the comparative process may be carried on, and made fruitful of valuable results.
- Whitney (1875), Pag. 314