This is the general condition among the mammals. It is the rule that the young are tended by the mother during the period of their youth. At birth they are usually helpless, and often are born before the eyelids have opened and while the body is yet naked, or but scantily clothed. But there are degrees of helplessness, determined, it would seem, by the conditions of the environment and habits of the parents. The maternal care is greater or less in accordance with the needs of the young. The period of youth is much longer, and increases as we ascend in the scale of life. The great apes, for instance—the gorilla, the orang and the chimpanzee—take from eight to twelve years to grow up, while baboons and common monkeys take from three to eight years, and the little South American monkeys and lemurs two to three years.[50] In connection with this longer childhood we find an increased mental growth; the years of youth are the time in which the brain cells increase in size and co-ordinate with the rest of the body. And the longer the period of youth the more perfect is the brain. Thus the helplessness of the young stands in direct relation to the increased vitality shown by the adults. It is also the strongest factor in developing and fixing the maternal instincts. The young do not leave their mother until they are well ready to start life on their own account; then they are thrown into the world. Till then they are cared for. Freed of any duty of finding food, and very seldom having to defend themselves, they have time to experiment and learn from experience. The instincts in this way become educated, their rigidity is destroyed, and more and more they are controlled by memory and experience—the stored-up results of experiment. The purpose of youth is to give time for this. The number of the young is now very greatly reduced, and the small families are protected by the mothers, in some cases assisted by the fathers. The maintenance of the species by the production of enormous families has ceased. Some of the small rodents, it is true, breed several times in the course of the year, and there are other fecund mammals, such as pigs, which give birth to many young in one litter. But these are rare exceptions. The usual number of young is two or three at a birth, and the higher in the scale of mammalian life the smaller is the family.[51]