The paragraph is followed by an incomplete phrase or sentence which summarizes the main idea of the paragraph. Choose the missing words of the phrase or sentence. 

There is generally a certain proportion between a living creature's length of life and the time it takes to grow to full size. There also exists some relationship between the growing body and the growing brain. The animals with more complicated bodies and brains develop more slowly, and are dependent longer upon their mothers than do creatures with simpler systems.
The （） of the growth of animals.