As stated above, all the families occupy the same basis of dissimilarity from one another—­i.e., none of them are related—­and consequently no two of them are either more or less alike than any other two, except in so far as mere coincidences and borrowed material may be said to constitute likeness and relationship.  Coincidences in the nature of superficial word resemblances are common in all languages of the world.  No matter how widely separated geographically two families of languages may be, no matter how unlike their vocabularies, how distinct their origin, some words may always be found which appear upon superficial examination to indicate relationship.  There is not a single Indian linguistic family, for instance, which does not contain words similar in sound, and more rarely similar in both sound and meaning, to words in English, Chinese, Hebrew, and other languages.  Not only do such resemblances exist, but they have been discovered and pointed out, not as mere adventitious similarities, but as proof of genetic relationship.  Borrowed linguistic material also appears in every family, tempting the unwary investigator into making false analogies and drawing erroneous conclusions.  Neither coincidences nor borrowed material, however, can be properly regarded as evidence of cognation.

While occupying the same plane of genetic dissimilarity, the families are by no means alike as regards either the extent of territory occupied, the number of tribes grouped under them respectively, or the number of languages and dialects of which they are composed.  Some of them cover wide areas, whose dimensions are stated in terms of latitude and longitude rather than by miles.  Others occupy so little space that the colors representing them are hardly discernible upon the map.  Some of them contain but a single tribe; others are represented by scores of tribes.  In the case of a few, the term “family” is commensurate with language, since there is but one language and no dialects.  In the case of others, their tribes spoke several languages, so distinct from one another as to be for the most part mutually unintelligible, and the languages shade into many dialects more or less diverse.

The map, designed primarily for the use of students who are engaged in investigating the Indians of the United States, was at first limited to this area; subsequently its scope was extended to include the whole of North America north of Mexico.  Such an extension of its plan was, indeed, almost necessary, since a number of important families, largely represented in the United States, are yet more largely represented in the territory to the north, and no adequate conception of the size and relative importance of such families as the Algonquian, Siouan, Salishan, Athapascan, and others can be had without including extralimital territory.