Urban growth is a form of metabolism.
There exists a life cycle, that begins from a center then concentrically expands like ripples, sometime coalescing into each other. The process does not terminate, but rather continues to create new ripples of expansion within the boundaries of the already established.
Transportation infrastructures set the limit of growth, and then the process begins to organize, disorganize and reorganize. Zones form, but in time, only to find their weak frames to spill into each other while another bursts out for the better.
Overflow and excess inundates the edges, a turmoil of change for the city to find its path. It is to ruin it all and then to build it all again one after the other, starting the ripples over again.
A form of organic growth is how Burgess seems to describe urban expansion. That there exists a general pattern of urban growth unique to geological locations and cities, and that is the urban form of life cycle.
Expansion, metabolism and mobility are key features that describe the process of urban expansion. Speed, in my opinion, is something that Burgess leaves out. Although he touches on the importance of contact, transportation, and mobility, I think the speed in which infrastructures and other key features operate within urban growth process have become a crucial factor of modern urban expansion. Not necessarily meaning that greater velocity equals a more rapid expansion, but the speed in which availability is brought to the people is a value that, in my opinion, may enrich a modern urban understanding.