The forebrain has considerably grown large during the course of vertebrate evolution because as more complex systems emerged in vertebrates, more primitive forms (traits) have been added instead of their removal or replacement. Thus, the elaboration of fore brain indicates the expansion of functions that are already present in the other brain regions. This made the brain to function at multiple levels.
The three functionally different regions of central nervous system (CNS) spinal cord, brain stem and forebrain indicate the evolution of multiple levels of function.
Thus, the blank can be filled with multiple levels of function.