I have quoted this hypothesis, as previously remarked, because it appears to me philologically interesting; but whatever may be thought of it by professional authorities, the evidence which the American continent furnishes of a polygenetic and polytypic origin of the native languages remains the same. And if there is good reason for concluding in favour of polygenetic origins of different types as regards the languages on that continent, of course the probability arises that radical differences of structure among languages of the Old World admit of being explained by their having been derived from similarly independent sources.[164] CHAPTER XIII. ROOTS OF LANGUAGE. In the last chapter my treatment of the classification and phylogeny of languages may have led the general reader to feel that philologists display extraordinary differences of opinion with regard to certain first principles of their science. I may, therefore, begin the present chapter by reminding such a reader that I have hitherto been concerned more with the differences of opinion than with the agreements. If one takes a general view of the progress of philological science since philology—almost in our own generation—first became a science, I think he must feel much more impressed by the amount of certainty which has been attained than by the amount of uncertainty which still remains. And the uncertainty which does remain is due rather to a backwardness of study than to differences of interpretation. When more is known about the structure and mutual relations of the polysynthetic tongues, it is probable that a better agreement will be arrived at touching the relation of their common type to that of isolating tongues on the one hand, and agglutinating on the other. But, be this as it may, even as matters stand at present, I think we have more reason to be surprised at the certainty which already attaches to the principles of philology, than at the uncertainty which occasionally arises in their applications to the comparatively unstudied branches of linguistic growth.