From this table there appears to be a steady decrease in the number of the deaf in relation to the general population from 1830 to 1860, this latter year seeming to be the low water mark. From 1860 to 1870 there is a slight increase, and from 1870 to 1880 a very large one, due to some extent to the method of taking the census. From 1880 to 1890 there is a certain decrease, though the proportion is still very high. From 1890 to 1900 there is a very considerable decrease, probably indicating a return to true conditions; and a not negligible decrease from 1900 to 1910. On the whole, with respect to these statistics, probably the most that we can safely say is that deafness is at least not on the increase relatively among the population, while there is a possibility that at present it is decreasing. For further determinations, we shall have to seek other means of inquiry. THE ADVENTITIOUSLY DEAF AND THE CONGENITALLY DEAF We may perhaps best approach the problem of deafness as an increasing or decreasing phenomenon in the population, if we think of the deaf as composed of two great classes: those adventitiously deaf, that is, those who have lost their hearing by some disease or accident occurring after birth, and those congenitally deaf, that is, those who have never had hearing.[17] In regard to the former class, it follows that we are largely interested in the consideration of those diseases, especially those of childhood, which may affect the hearing, and in their prevention or diminution we can endeavor to ascertain how far there are possibilities of reducing the number of the deaf of this class. In the latter case we are called upon to examine some of the great problems involved in the study of heredity, especially in respect to the extent that the offspring is affected by defects or abnormalities of the parent, and to see what, if any, means are at hand to alter conditions that bring about this form of deafness. We shall first discuss the causes of adventitious deafness, together with the possibilities of its prevention and the likelihood of its diminution, and then consider the questions involved in congenital deafness. ADVENTITIOUS DEAFNESS AND ITS CAUSES From three-fifths to two-thirds of the cases of deafness are caused adventitiously--by accident or disease. To accidents, however, only a very small part are due, probably less than one-fiftieth of the entire number.[18] Nearly all adventitious deafness results from some disease, either as a primary disease of the auditory organs, or as a sequence or product of some disease of the system, often one of infectious character, the deafness thus constituting a secondary malady or ailment. The larger portion is of the latter type, probably less than a fourth resulting from original ear troubles.[19] In either case deafness occurs usually in infancy or childhood, and does its harm by attacking the middle or internal ear.