A résumé of Mitchell’s studies indicates that the higher apes have a potential longevity and a hardihood much less than man but still upwards of thirty years. As one investigates lower in monkeys, life periods become shorter, while in the next order, Lemures, the length of life rises. Carnivora have a reasonably good vitality, their potential periods varying from ten years in the foxes to thirty-three years in bears. Insect eating animals are short lived, three years being a maximum. The Bat family shows great variations, the greatest life being not over seventeen years. The Rodentia have long lives compared to their sizes—twenty years in porcupines, fifteen years in squirrels, thirteen in marmots, nine in agoutis and capybaras, and three in dormice (which is also about the maximum for the rat). Hyraces live four years on the average. Proboscidea, although reputed to live to great age, probably rarely live a half century and may be said to have an expectancy of twenty to thirty years. Perissodactyla (horses, tapirs and rhinoceroses) while they may live half a century, have an average life of between fifteen and thirty years. The closely related Artiodactyla fall into two groups, a first comprising antelopes, sheep, goats and deer which rarely exceed seventeen years, and a second consisting of cattle, camels and giraffes, which vary in expectancy from eighteen to thirty years. The smaller members of the Ungulata have in relation to size a relatively greater viability, the ruminants, however, having on the whole a low viability. Marsupials vary from a maximum of seven years in the opossum to eighteen in the wombats, but none of this group has a good viability. The Aves as a class or if compared according to dietary requirements, have longer potential ages and better viability than mammals. Passerine birds average twenty years and many live to sixty, while the Picariæ approach the former figure but do not have such good viability. Psittaci and Striges may live a half century but the resistance of the latter is much reduced by any unfavorable surroundings. The raptatory birds live fifty years, but their viability is variable. Herodiones have a maximum expectancy of thirty years and good resistance, while their relatives, Steganopodes, may live fifty years, and Odontoglossæ have a good viability, up to twenty years. Anserine birds may live to be fifty, and, unless conditions are quite unfavorable, have a good resistance. Columbæ may under good conditions live to be fifty. Gallinaceous birds may only be expected to survive twenty years, a figure also given for Fulicariæ. Alectorides may live up to fifty years. Limicolæ, though they do not thrive in captivity, may live thirty years. Impennes live poorly under artificial conditions, the greatest record being twelve years, a figure also holding for Crypturi. Struthiones, if the conditions be right, may live fifty years.