The distribution of certain genera is ex tremely restricted. The genus Romerolagus, an aberrant rabbit having but a single species with out near living relatives, is confined to a narrow belt about the bases of the adjacent volcanoes Popocatapetl and Iztaccihuatl on the border of the valley of Mexico, although exactly similar slopes with the same soil and vegetation exist on other mountains distant only a few miles. Another example is the curious rodent genus Aplodontia, confined to damp forests of the northwestern United States and represented by a single species. Such mammals are, no doubt, survivors of groups once having far wider dis tribution and more varied forms, and their present restriction in variety of form and ex tent of range is a forecast of approaching ex tinction. The llamas and alpacas of South America are similar survivals of once widely spread types. In strong contrast to such re stricted distribution may be cited other genera which contain many forms, abound in indi viduals and occupy vast areas of the earth, thus indicating a vitality and attendant fecundity as well as a youthful plasticity of organization able promptly to respond to the requirements of changing conditions. The genus Leptis, which includes the hares, is perhaps the most notable of such groups. It has representatives throughout most of Africa, Europe, Asia and North America, including in its range the northern end of Greenland and the tropic low lands. The meadow mice of the genus Micro tus, numbering many species and untold mil lions of individuals, occupy most of the low lands and mountain ranges of Europe, the northern half of Asia and America north of Mexico. The vast range. of the hardy puma (Felts concolor) has already been mentioned. The disappearance from the earth of once abundant and widely spread species of mammals when climatic and other conditions appear to remain the same presents an unsolved problem. The mammoth, cave bear, and other large species now extinct, ranged the plains of northern Europe and Asia as late as the time of the Cave Men, while the reindeer and other animals contemporaneous with those named and apparently no better fitted for existence still survive. It has been wisely suggested by Ly dekker as probable, among the many factors which influence the disappearence of species and even of larger groups of mammals and other forms of animal life, that each species or even much more important groups may, like the in dividual, have its period of youth, maturity and decay, leading to loss of vitality and final extinction, to be replaced by vigorous new comers in the endless succession of life. The reasonableness of the foregoing surmise is shown by the facility with which certain species of birds and mammals have been known to decrease and even disappear within a very re cent period. One striking instance within the last 30 years is the inexplicable disappearance of various species of birds in the Hawaiian Islands with no change in their habitat sufficient even remotely to account for it. Their extinc tion appears to have been due to a lack of enough vitality to withstand the trifling en croachment on the great expanse of their forest homes following the advent of civilized man.
The steadily decreasing numbers of the prong horned antelope on the open plains of the west ern United States and northern Mexico is no doubt in part due to its small powers of resist ance to changes in environment brought by the increased occupation of its territory by man and domestie animals. The lack of ability to resist changes on the part of the prong-horn is strikingly shown by the difficulty experienced in keeping them alive in zoological parks and to maintain small herds of them in fenced govern ment game preserves containing thousands of acres in the midst of their former haunts. In these same .parks and preserves, buffalo, elk and deer thrive and increase rapidly.
Man appears to have originated in the Old World and to have become the most widely dis tributed of all mammal types. The genus Homo has broken up into a considerable number of specific types which had originally as clearly defined geographic distribution as the lowest of mammals. With the development of civilization man has more and more tended to lose these local distinctions and to obscure the original boundaries of races by migrations at will to all parts of the earth. The weaker species of man disappear before the stronger, as occurs among other forms of animal life.
No treatment of the distribution of recent mammals can well avoid the consideration of the potent influence of man. Although man is the latest type of mammal to appear on the earth he rapidly developed a dominant position and spread to all lands where he has determined the survival or destruction of numerous species, and his influence in this re spect is rapidly increasing. From the most primitive times up to the present animal products, including flesh, fat, hides, ivory and others, have been necessary to man's welfare and have formed valued articles of commerce which has led to persistent pursuit and slaughter. In addition man has domesticated the horse, cattle, sheep, goats, pigs, dogs and cats, which he has taken with him to other lands where under his protection they have enormously increased to the injury of most native mammals, both great and small, with which they have come in competition. The present situation in the United States is an illustration of this in the presence of many millions of domestic animals, and the vast de crease in all the larger game animals, such as bison, elk, deer, antelope and mountain sheep, since the settlement of the country. Not all, by far, of this harmful influence on the native mammal life can be laid to the domestic ani mals, for the improvement of weapons of the chase and of means of transportation coupled with the hunting instinct which persists in man are ever-increasing menaces to the welfare of the surviving large and small mammals which offer any value as game, as bearers of fur or as the producers of commercial products. This danger extends to the species living in the sea as well as to those on land. The increasing occupation and cultivation of the land is another of the factors introduced by man whereby the existence and orderly development of the higher forms of life on the earth is being more and more disturbed and in many instances jeop ardized.