Among local catastrophes brought about by unusually prolonged cold may be cited the decimation of the fur-seal herds of the Pribilof Islands in 1834 and 1859, when the breeding seals were prevented from landing by the presence of ice-floes, and perished by thousands. Peculiar interest is attached to this case, because the restriction of the northern fur-seals to a few isolated, long undiscovered islands, is believed to have been brought about by their complete extermination in other localities by prehistoric man. Had these two seasons killed all the seals, it would have been a reversal of the customary extermination by man of a species reduced in numbers by nature. In the case of large animals another element probably played a part. The larger the animal, the fewer young, as a rule, does it bring forth at a birth, the longer are the intervals between births, and the slower the growth of the young. The loss of two or three broods of sparrows or two or three litters of rabbits makes comparatively little difference, as the loss is soon supplied, but the death of the young of the larger and higher mammals is a more serious matter. A factor that has probably played an important role in the extinction of animals is the relation that exists between various animals, and the relations that also exist between animals and plants, so that the existence of one is dependent on that of another. Thus no group of living beings, plants or animals, can be affected without in some way affecting others, so that the injury or destruction of some plant may result in serious harm to some animal. Nearly everyone is familiar with the classic example given by Darwin of the effect of cats on the growth of red clover. This plant is fertilized by bumble bees only, and if the field mice, which destroy the nests of the bees, were not kept in check by cats, or other small carnivores, their increase would lessen the numbers of the bees and this in turn would cause a dearth of clover. The yuccas present a still more wonderful example of the dependence of plants on animals, for their existence hangs on that of a small moth whose peculiar structure and habits bring about the fertilization of the flower. The two probably developed side by side until their present state of inter-dependence was reached, when the extinction of the one would probably bring about that of the other. It is this inter-dependence of living things that makes the outcome of any direct interference with the natural order of things more or less problematical, and sometimes brings about results quite different from what were expected or intended. The gamekeepers on the grouse moors of Scotland systematically killed off all birds of prey because they caught some of the grouse, but this is believed to have caused far more harm than good through permitting weak and sickly birds, that would otherwise have fallen a prey to hawks, to live and disseminate the grouse distemper.