The central theme that emerges during this poem deals with our accepted ideas about the process of evolution and our own place in it.
The encounter with this enormous, almost primeval beast at the opening of the poem acts as a catalyst to consider the relationship between this creature and humans, and the comparative paths such differing species took.
On one hand, basking sharks, have remained relatively unchanged for millions of years, while on the other, humans have vastly changed since the days when marine life first crawled ashore and adapted to a life on land.
Through the reflection of the speaker, we are reminded that we have much more in common with the shark than we may initially believe, and by rewinding time back to the origins of evolution itself he creates a direct link between it and us. In doing so, he forces us to revise our understanding of exactly what we mean when we use the term
monster, suggesting it is humanity and not creatures like the shark that are capable of true monstrosity.