上周提起，我刚从澳大利亚回来。在那儿，我参观了一片桉树林，那片树林二月刚刚遭受一次骇人的野火肆虐。也许是因为我还有些天真，曾以为会发现许多树被烧死。但它们却没死。树皮烧黑了，不过看上去还好，有许多树还长出了新的嫩叶。这种现象让我想到了火与它作为一种自然力的作用。

As I mentioned last week, I’ve recently returned from Australia. While I was there, I visited a eucalyptus forest that, in February, was the scene of an appalling wildfire. Perhaps naively, I had expected to find that many trees had been killed. They hadn’t. They had blackened bark, but were otherwise looking rather well, many of them wreathed in new young leaves. This prompted me to consider fire and the role it plays as a force of nature.

木炭化石告诉我们，自陆地上有了植物，野火就一直是地球上生物的一部分，也就是说，火的历史要在4亿年以上。早在草这类生长迅猛的植物存在之前，火就已经存在了；而在第一束花朵出现之前也早已有了火。不必把火想得很神秘，火，在很多方面，就像是一种动物，尽管飘忽不定、难以捉摸。和其他动物一样，火消耗氧气，像一只绵羊或蛞蝓一样吞食植物。但和正常动物又不同，有着捉摸不定的形态。有时仅仅轻咬几片叶子，有时却能吞噬参天大树。有时比蝗灾更致命，破坏性更大。

Fossil charcoals tell us that wildfires have been part of life on Earth for as long as there have been plants on land. That’s more than 400 million years of fire. Fire was here long before arriviste plants like grasses; it pre-dated the first flowers. And without wanting to get mystical about it, fire is, in many respects, a kind of animal, albeit an ethereal one. Like any animal, it consumes oxygen. Like a sheep or a slug, it eats plants. But unlike a normal animal, it’s a shape-shifter. Sometimes, it merely nibbles a few leaves; sometimes it kills grown trees. Sometimes it is more deadly and destructive than a swarm of locusts.

火的这种形态捉摸不定的性质让研究更加困难了，因为它并非单一的整体。有些火炙热可怕；有些火则温度相对较低。有些火在地面上燃烧；另一些则会窜上大树。此外，世界上有些地方更容易出现火灾。地球的卫星图像显示，欧洲北部很少出现野火，而在非洲中部的一些地方和澳大利亚则常有野火。（近年来很多野外火灾是人为的，要么是故意，要么是偶然。但在我们的祖先扔火把或烟蒂之前的很长一段时间，火灾是由闪电或雪崩中乱石摩擦引起的。）

The shape-shifting nature of fire makes it hard to study, for it is not a single entity. Some fires are infernally hot; others, relatively cool. Some stay at ground level; others climb trees. Moreover, fire is much more likely to appear in some parts of the world than in others. Satellite images of the Earth show that wildfires are rare in, say, northern Europe, and common in parts of central Africa and Australia. (These days many wildfires are started by humans, either on purpose or by accident. But long before our ancestors began to throw torches or cigarette butts, fires were started by lightning strikes, or by sparks given off when rocks rub together in an avalanche.)

一旦起火，就会有许多影响火势的因素。显然，天气对火势有着极大的影响：风助火势、雨灭火威。火势也受地形的影响：火易顺山而上，而非沿坡向下。另一个关键因素就是：火易吞噬哪类植物。

Once a fire gets started, many factors contribute to how it will behave. The weather obviously has a huge effect: winds can fan flames, rains can quench them. The lie of the land matters, too: fire runs uphill more readily than it goes down. But another crucial factor is what type of plants the fire has to eat.

有一项常识：经常遭大火肆虐的植物具备助于它们应对火灾的特性——如厚厚的树皮或只有经过烈火和烟熏才会生长的种子。但人们并不太注意，这些植物本身就影响了火的性质及火势。

It’s common knowledge that plants regularly exposed to fire tend to have features that help them cope with it — such as thick bark, or seeds that only grow after being exposed to intense heat or smoke. But what is less often remarked on is that the plants themselves affect the nature and severity of fire.

例如，死掉的树枝比活着的着火更快，所以留着死枝（而非让它们落下）的树更易让火焰爬上森林的树冠：死去的枝条做了火焰的梯子。枯枝也让火焰更加炙热。纤维素含量高或含油的叶子也助长了火势。树脂和树胶也非常易燃。每个女童子军队员都知道，细枝比粗枝更容易着火，所以细枝多的植物比枝条不那么细的同类姐妹更容易着火。

For example, dead branches burn more readily than living branches, so a tree that keeps dead branches (rather than letting them fall) makes it easier for a fire to climb into a forest canopy: the dead branches provide a ladder for the fire. Deadwood also allows fires to get hotter. Leaves that are high in cellulose, or that contain oils, also stoke the flames. Resins and gums are highly flammable. And as any girl scout knows, twigs catch light more readily than branches, so a twiggy sort of plant can catch fire more readily than its non-twiggy sister.

但有件古怪的事：很多长在易着火地方的植物非常易燃——比长在其他地方的植物更易燃。这一事实让人推测：这些植物已进化得会导致火灾了，也就是说它们“想要”火，并进化出了让火星更易变成火焰、火焰更易变成火灾的特征。我称之为“点燃我”假设。

But here’s the odd thing. Many plants that live in places prone to fire are highly flammable — more flammable than plants that live elsewhere. This has led some to speculate that these plants have actually evolved to cause fires: that they “want” fire, and have evolved features that make it more likely that a spark will become a flame, and a flame will become a fire. I call this the torch-me hypothesis.

论证如下：很多植物依靠火焰繁殖。事实上没有火这些植物就会消亡。例如，假如长叶松林不经常着火，这些松树就会被水橡树和其他物种取代。因此——继续推断——火灾是有价值的，因为它们消灭了竞争。助燃的植物或许会因此拥有进化优势：在为自己的种子创造适宜发芽的环境的同时，它们破坏了竞争。

The argument goes like this. Many plants depend on fire for their propagation. Indeed, without fire, these plants disappear. If, for example, longleaf pine forests do not burn regularly, the pines will be replaced by water oaks and other species. So — runs the argument — fires are desirable because they kill the competition. Plants that enhance fires may thus have an evolutionary advantage: they murder the competition while creating the right circumstances for their own seeds to sprout.

该设想引起了热烈的辩论。问题在于，证明某项特性因其助燃而进化产生很困难。的确，含油的叶子更易燃；但或许含油叶子的真正优势在于昆虫不爱吃它们。这样，它们的可燃性或许就是糟糕味道的副产品了。

This idea has sparked a heated debate. The problem is, showing that a trait has evolved because it enhances fire is difficult. Yes, oily leaves are more flammable; but perhaps the real advantage of oily leaves is that insects don’t enjoy eating them. Then, their flammability may be a by-product of tasting terrible.

植物在进化中变得能助燃的最佳证据来自松树。某些种类的松树保留死去的枝条；其他的则抛弃自己的枯枝。正如你基于“点燃我”假设所期待的，更易燃的树种——保留枯枝的树——也会长出依靠火来发射的种子。简言之，这两项特性合而为一了。

The best evidence that some plants may have evolved to promote fire comes from pines. Some species of pine keep their dead branches; others tend to self-prune. As you would expect under the torch-me hypothesis, the more flammable species — the ones with the dead wood — also tend to have seeds that are released by fire. In short, the two traits go together.

这一证据具有启发性。但由于没有其他植物的更多数据支持，这一证据仍很单薄。如果把树放在被告席上，指控它为纵火罪的同谋，陪审团目前会不得不驳回判决，因为“尚未证明”。但当我想起澳大利亚那片广阔的桉树林，我还是禁不住遐想：树的进化真的让火势更加猛烈了吗？

Which is suggestive. But without more data from other plants, the evidence remains thin. If a tree were put in the dock and charged with being an accessory to arson, the jury would, for now, have to return the verdict, “Not proven.” Yet as I think of that great eucalyptus forest in Australia, I can’t help wondering. Have the trees actually evolved to make the fires worse?