I have just heard thrilling news about the most endangered habitat on Earth: a habitat I was exploring, on horseback and on foot, a few weeks ago. There are signs that a series of new chunks of it can be saved, protected, cherished: and that the howler monkeys will continue howling and the jaguars will continue to lurk.

I made the trip with the World Land Trust, which specialises in the buying-up of endangered habitats. And here’s the good news: a complex series of talks and meetings have been set up with a view to buying up crucial areas – wildlife corridors – of the Atlantic rainforest in Brazil. There’s just 4 per cent of it left.

Here’s the snag: you don’t march into a foreign country and start buying it up. That’s neocolonialism and a Bad Thing. And you can’t buy an area of forest and consider that you’ve done a good job. The place needs to be visited, patrolled and looked after: otherwise poachers of both animal and trees will move in, and others will simply take the place over.

It’s a ticklish business, involving the funding of a local NGO for the purchase, and subsequent advice on development and maintenance. I paid a visit to the Reserva Ecológica di Guapi Assu, where purchases, aided by WLT, have already been made and stunning areas of forest have been made safe. That’s where I climbed slopes so steep a man would need to use his hands in places – but my horse, a skinny beast of bottomless stamina, marched up with a bounce in his stride between the soaring pillars of the trees beneath the vaulted roof of the canopy. It’s impossible to enter either rainforest or a cathedral without a feeling of reverence. It’s not because a forest is like a cathedral but because a cathedral is like a forest. And then down south to save the monkey-puzzle tree. The forest here is dominated by araucarias, which are related to the gloriously silly trees you find in English gardens. But these are fantastically enlarged, and, in maturity, look like megalomaniacal cow-parsley.

I did genuflect as I entered: though to inspect an ocelot scat. There was the burrow of an armadillo, a place where the howlers howl, a pool with butterflies of quite astonishing beauty and diversity. But never mind all that: here, the forest itself is the star: a place of epic silences, for though the place is species-rich, it has always been population-poor. Which makes it the more fragile.

But as you walk the forest paths – snaking vines, moss-laden trunks, branches bearing impossible burdens of bromeliads - it is the strength of the place that gets to you: its integrity, its endlessness.

All this araucaria forest is in private hands. It is illegal to destroy it, but enforcement is another matter; and, besides, governments and policies change. But purchase would put it beyond the reach of changing fashion and give the forest back to itself.

The surviving 4 per cent of Atlantic rainforest is still breathtakingly impressive, as I witnessed on the most thrilling train ride in the world, the Serra Verde Express from Curitiba to Morretes. Here, the train clings like a fly to the wall of these melodramatically forested hills: endless green acres, and every one of them worth saving. To look out at this impossible vista is to fill the heart. For, even in the guts of the most damaged habitat in earth, there is something left, something worth keeping, something on which a hope can be constructed.

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