In Aspects of the Novel, E.M. Forester describes the form as a kind of river bounded by two chains of mountains, poetry and history, that empties into the sea, which is time. Like all good metaphors, this one can be expanded or contracted according to the taste of the reader. There are more parts to this river (perhaps an infinite number) to be enumerated and explicated.
The river of the novel has a source: myth, where fear and wonder join themselves in the first act of creation. On the path towards the sea, it passes through numerous bends, breaches, sloughs and oxbows. With each change in course, the river tends towards one range of mountains or the other, this time towards the practice of history, that time towards the writing of poetry. Sometimes the river cuts a deep canyon straight between the two of them.
And there is not one river but many, each with its own course that brings it through vague territories of language, nation, and custom. A river can be diverted, dredged, dammed, bridged or drained completely. Some rivers are better used than others. Some rivers never reach the sea.
And then there are the fish, who are, naturally enough, the authors, each species with its particular niche. There are trout, who prefer the sober clarity of a mountain stream. There are tetra, brilliant but disposable. There are sturgeon, who skulk in the depths and possess an encyclopedic diet.
Some fish are more energetic than others. Each year the salmon leave the ocean and swim towards the source of the river. There they spawn and die. There are lampreys and there are piranhas in the river. Carp, catfish, and gar. And there are the dolphins, who can live and thrive in the river but are not really fish. They are the happiest. There are manatees in the river as well.