Mr. Espinoza, a first-grade teacher, tells his student-teacher that when the children begin school they seem very immature; that is, they cannot yet read, write, or do simple calculations, nor can they maintain attention for long periods of time. However, at the end of the school year they seem more mature; that is, they can read, write, and do simple calculations, and they can maintain attention for long periods of time. Mr. Espinoza is referring to what aspect of development?