Oh dear. I seem to have completed my life's work. How very odd. I've just seen a tiger, you see. What on earth can you do after that? And not only is a tiger perhaps the ultimate wildlife sighting: it is also my fourth and final species from the genus Panthera. That is to say, the group that contains the proper big cats. Where do I go from here?

But let me tell you about my tiger. I was in India last week saving the elephants, an account of which you will find elsewhere in this newspaper. I was riding an elephant rather than saving it, and we plunged into a sea of grass, the howdah lurching and swaying to the complex rhythm of the elephantine stride and the mahout urging the elephant on with detached gentleness.

A long, low-speed pursuit through the grass, and me already braced for disappointment. I couldn't believe that this crowning moment would come. But suddenly we were more or less on top of it: the tiger crouching in the grass, the body curved sideways and away from us in defensive threat, the state in which a cornered cat will sometimes fly at a dog.

Sometimes they fly at elephants. Put tiger attack into YouTube and you can watch a tiger springing 11ft up in the air to attack a group of conservationists on elephant back - a chunk of footage that my conservationist friends had obligingly shown me before we set off on this jaunt.

But the colour of it - glowing as if lit from within. The snarling anger of it. The movement, as if every joint has been bathed in a gallon of oil. And it turned and galloped on huge soft paws through the grass, a striped thing in a striped world, off to find a place where elephants are less intrusive, leaving the elephant's passengers almost as luminous as the tiger himself - lit up from within with excitement and joy. David Bebber, Times photographer on the elephant behind me, had only a lightning-quick view but he was sharp enough to catch our tiger for all time.

Feasting with panthers

I have no more Panthera to look for, no more big cats to find, no new worlds to conquer. I have seen lions on many occasions in Africa: the first were in the South Luangwa National Park in Zambia, polishing off the last bits of a buffalo they had killed. Later I sat on the banks of the river drinking a Mosi beer while the lions sat a couple of hundred yards away; all of us dangling our paws over the edge and contemplating life with immense satisfaction.

I saw my first leopard on that same trip, making a night hunt through the stripes and shadows of an ebony glade, caught in a spotlight, moving like the smoothest thing anybody could ever think of, eventually bringing down an impala and then, with impossible strength for something so graceful, galloping straight up a tree trunk with the impala in its jaws, there to sit in the crotch of a branch and contemplate its own achievement. I saw jaguar in Belize - a vision of strength and power strolling at his ease along a forest road: the most elusive member of the genus, a forest creature far better at hiding than being seen. Even those who live in the forest see a jaguar only once or twice a year; I was there five days and was given the luck no one can ever earn.

And, er, that's it. Unless, of course, scientific consensus places the snow leopard firmly in the Panthera group - it is usually put in a genus of its own. The clouded leopard is also in a different genus. So that's it.

The four great panthers of the world, each one a thing of beauty, each one a thing of perfection, each one the most glorious and perfect thing anybody could ever wish to see - and I, who wished above all to see them, have now seen every one of them.

My to-see list

But on the whole and taking one thing with another, I don't think I'll be hanging up my binoculars just yet. There are other thrilling beasts to look for. I have seen only two species of bear, the same number of great whales. I may have worked my way through the sexiest genus of them all, but there are a million more marvels still unseen. That, for the human observer at least, is what biodiversity means.

But even if I only saw the same things over and over again, I'd be more than content. It really isn't all about ticking things off a list. Could anyone ever be sated with panthers? Oh, don't let's bother looking at that tiger, I've seen more than enough. All the same, there is a profound joy in this completion. I feel, in a strange way, omniscient. But the wild world is so vast and so complex that I can also take profound satisfaction in the certainty that I will be ignorant again tomorrow.

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