Although each baby has an individual schedule of development, general patterns of growth have been observed. Three periods of development have been identified (确定) , including early infancy (幼儿期) , which extends from the first to the sixth month middle infancy, from the sixth to the ninth month and late infancy, from the ninth to the fifteenth month. Whereas the newborn is concerned with his or her inner world and reacts primarily to hunger and pain in early infancy, the baby is already aware of the surrounding world. During the second month, many infants are awake more and can raise their heads to look at things. They also begin to smile at people. By four months, the baby is searching for things but not yet grasping them with its hands. It is also beginning to be wary(谨慎的) of strangers and may scream when a visiting relative tries to pick it up. By five months, the baby is grabbing objects and putting them into its mouth. Some babies are trying to feed themselves with their hands. In middle infancy, the baby concentrates on practicing a great many speech sounds. It loves to imitate actions and examine interesting objects. At about seven months, it begins to crawl, a skill that it masters at the end of middle infancy. In late infancy, the baby takes an interest in games, songs, and even books. Progress toward walking moves through standing, balancing, bouncing in place, and walking with others. As soon as the baby walks well alone, it has passed from infancy into the active toddler (蹒跚学步) stage. What is the main subject of this reading passage?