_Memory_ is the power of retaining or recognising past states of consciousness. The power to retain impressions follows in origin close upon the power to receive impressions. Memory is the historic faculty of the mind—the power of the mind to store up its experiences—and is found in nearly all animals. The lowly limpet, whose world is a seaside rock, will come back from its little roamings time after time to the same rude lodge from which it set out. Bees remember where they get honey or sugar months afterwards, and when it is necessary will sometimes go back to the old home hive which they left the year before. Ants retrace their steps after making long journeys from their nest, and are able in some way to recognise their friends after months of separation. The stickleback (fish) knows the way back to his nest, although he has been absent several hours. Fishes return and hatch their young year after year in the same waters; birds come back to their old nesting-places; and horses remember their way along devious roads over which they have not been for years. Horses used in the delivery of milk, or in other occupations in which they are accustomed to travel daily over about the same route, come in time to remember every alley, street, and stopping-place of the whole round almost as accurately as their drivers. Darwin’s dog remembered and obeyed him after an absence of five years. The power of dogs, squirrels, and other animals of remembering where they have long before cached food is indeed wonderful. A squirrel will come down out of a tree when the earth is covered to a depth of several inches with lately fallen snow and hop away, without the slightest hesitancy or mistake, to the exact spot where it has months before stored its mid-winter acorns. A lion has been known to recognise its keeper after seven years of separation, and an elephant obeyed all his old words of command on being recaptured after fifteen years of jungle life. The similarity of memory in other animals to the same faculty in man is shown by the fact that memory everywhere is governed by the same laws. In all animals, including man, memory is strengthened by repetition—that is, impressions are always deepened and confirmed by being made over and over. A parrot or a raven masters a new sentence by working at it and saying it over and over again, just as a boy memorises his rules and catechisms.