Answer: the principle of hierarchical integration.
Explanation: The principle of hierarchical integration in child development explains that simple skills are developed separately and independently then overtime, they are integrated into more complex skills.
Like in the question above, Sherri at 3 months old can hold the rattle but not quite comfortably (simple skill developing independently). At 11 month, she can hold comfortably and shake the rattle (simple skill developed into more complex skill).
