What is the role of nature in "A Passage to India"?

Nature in A Passage to India is used in building atmosphere and in symbolism.

Forster refers to nature continuously throughout the novel, and often to build the atmosphere of a location or event. For example, in the very beginning of the novel, describing Chandrapore, a fictional city in India, Forster consistently uses natural objects to describe the city: the Marabar Caves, the River Ganges, bank, stream, the sea, filth, mud, soil; in fact at the end of the first paragraph Chandrapore is described as something that itself appears to be alive: "...the general outline of the town persists, swelling here, shrinking there, like some low but indestructible form of life." This constant reference to nature establishes that in India, there are no boundaries between urban and rural areas and that the environment in India, both natural and social, is unforgiving.

In Part II - Caves, Forster uses several paragraphs to detail the ancient natural history of India. This makes the entire country feel like a living, evolving entity, and renders the human lives upon it momentary and insignificant.

In the days of the prehistoric ocean the southern part of the peninsula already existed, and the high places of Dravidia have been land since land began, and have seen on the one side the sinking of a continent that joined them to Africa, and on the other the upheaval of the Himalayas from a sea. They are older than anything in the world. No water has ever covered them, and the sun who has watched them for countless aeons may still discern in their outlines forms that were his before our globe was torn from his bosom.

The sun mentioned in this passage continues to be referenced the whole time Mrs. Moore, Adele, Aziz, and the others are at the caves, establishing an atmosphere of bright, sweltering uncertainty.

An example of nature being used symbolically in this novel is the wasp. The wasp is mentioned three times in “A Passage to India,” making it both a symbol and a motif. In Part I - Mosque, Chapter 3, Mrs. Moore encounters a wasp while hanging up her coat: "Going to hang up her cloak, she found that the tip of the peg was occupied by a small wasp." Mrs. Moore notes that the Indian wasp is different from the British wasp, echoing the novel's theme of the differences between the Indian people and the British occupiers. Mrs. Moore contemplates the wasp: nature symbolizes the blurred lines between interior and exterior spaces in India. The wasp here specifically symbolizes the tendency of nature to intrude into interior spaces in India:

Perhaps he mistook the peg for a branch--no Indian animal has any sense of an interior. Bats, rats, birds, insects will as soon nest inside a house as out; it is to them a normal growth of the eternal jungle, which alternately produces houses trees, houses trees.

Nature is used symbolically in a conversation in Chapter 4, between two missionaries -- Mr. Graysford and Mr. Sorley -- in which they discuss the idea that all God's creations are equal and welcome in their house. The missionaries mention several examples from nature, in descending order of status: monkeys, jackals, wasps, oranges, cactuses, crystals, mud, and bacteria. Mr. Sorley believes that all except bacteria belong in their gathering: "No, no, this is going too far. We must exclude someone from our gathering, or we shall be left with nothing." By excluding bacteria, Mr. Sorley symbolically refers to the line that is drawn between whom the British include and exclude from their gatherings.

We meet the wasp again in Part III - Temple, Chapter 33, when Godbole is dancing and meditating. He recalls Mrs. Moore and that he loves her, and he next recalls a small wasp and realizes that he loves the wasp, as well, as God loves all His creatures. Then Godbole attempts to love the stone the wasp sat upon, but is not able to. Godbole also draws a line, this time between what is included in his love and what is not.