This is because both the last-named subjects are necessary as preliminaries. The structure of the language supplies us with the points in which one dialect may differ from another, whilst the history of the immigrant populations may furnish an ethnological reason for such differences as are found to occur.
For a further illustration of this see pp. [4], [5].
[§ 679]. By putting together the history of the migrations into a country, and the grammatical structure of the language which they introduced, we find that there are two methods of classifying the dialects. These may be called the ethnological, and the structural methods.
According to the former, we place in the same class those dialects which were introduced by the same section of immigrants. Thus, a body of Germans, starting from the same part of Germany, and belonging to the same section of the Germanic population, even if, whilst at sea, they separated into two, three, or more divisions, and landed upon widely separated portions of Great Britain, would introduce dialects which were allied ethnologically; even though, by one of them changing rapidly, and the others not changing at all, they might, in their external characters, differ from each other, and agree with dialects of a different introduction. Hence, the ethnological principle is essentially historical, and
is based upon the idea of affiliation or affinity in the way of descent.
The structural principle is different. Two dialects introduced by different sections (perhaps it would be better to say sub-sections) of an immigrant population may suffer similar changes; e. g., they may lose the same inflexions, adopt similar euphonic processes, or incorporate the same words. In this case, their external characters become mutually alike. Hence, if we take two (or move) such dialects, and place them in the same class, we do so simply because they are alike; not because they are affiliated.
Such are the two chief principles of classification. Generally, they coincide; in other words, similarity of external characters is primâ facie evidence of affinity in the way of affiliation, identity of origin being the safest assumption in the way of cause; whilst identity of origin is generally a sufficient ground for calculating upon similarity of external form; such being, a priori, its probable effect.
Still, the evidence of one in favour of the other is only primâ facie evidence. Dialects of the same origin may grow unlike; dialects of different origins alike.
[§ 680]. The causes, then, which determine those minute differences of language, which go by the name of dialects are twofold.—1. Original difference; 2. Subsequent change.
[§ 681]. The original difference between the two sections (or sub-sections) of an immigrant population are referable to either—1. Difference of locality in respect to the portion of the country from which they originated; or 2. Difference in the date of the invasion.