Recently, and for the first time, I flew right across Australia. Heading northwest from New Zealand, I crossed Australia's southeast coast somewhere south of Sydney and traversed the country northwest to the coast near Broome.

For hour after hour the scenery rolled beneath the aircraft, a seemingly endless sea of reds and oranges, hundreds of elongated ridges in parallel, dried-up river courses, saline lakes almost as dark as the surrounding soil, and hardly anywhere any sign of vegetation.

The huge red heart of Australia was dotted with shadows of tiny clouds, resembling fluffy tufts of cotton wool as they slid tantalizingly over the parched landscape. Australia's interior has so little change in elevation that there is just nothing to trigger those flimsy wisps of water vapor to swell and rise and deliver their moisture to the Earth.

Travel on toward the coast though, and over a narrow rim of land the clouds increasingly brew and tower. Down below, the hints of green coalesce into a landscape that looks almost liveable. The heart of the vast continental desert is left behind, and its harshness fades to a rather Mediterranean coastal climate. Still warm, still dry, this is a far more tolerable environment for life. The deep oranges and reds of the desert, the bare bones and skin of the planet, are thinly coated here with a flimsy coating of green.

Continuing northwest, sparkling ocean separates the innumerable islands of the Indonesian archipelago. The fine patina of green that marks the coastal zone of Australia is now traded for a solid carpet of color. In this region the temperatures, though high, are less searingly dangerous to plant life.

The air here is palpable in its humidity; it carries a heavy scent of life. Summer and winter barely register in the annual parade of temperatures; in fact, there is greater variation between night- and daytime temperatures than there is between the seasons. In this relatively stable climate, the long slow march of evolutionary time has allowed mangroves and tropical rain forests to form the natural climax vegetation. From the desert heart of Australia to the lushly clad mountains of Sumatra is just a few hours by plane, yet in ecological terms they are light years apart.

To travel from the stony desert to the tropical forest is to move from one extreme environment, one extreme of climate and of biodiversity, to its opposite, but not all regions epitomize such extremes.

Sandwiched between the Arctic and the tropics there lies a region where the temperature and humidity are neither too low nor too high, where a cyclic pattern of seasons brings an attractive variety to the year. In this region, biodiversity is neither as limited as it is in the desertlike Arctic or a continental desert, nor is it as prolific as in the rain forests of the tropics. It lies somewhere comfortingly in between.

We call the climate here "temperate," and the huge area that experiences this climate the "temperate region."

Much of North America, much of Europe, and a broad band of Asia lie within the temperate region. Asia's temperate zone stretches from the flanks of the Himalayas in the west to the central islands of Japan in the east. A quirk of geological history means that the temperate region of Japan, particularly of Honshu, supports a staggering array of plant life.

Rotating around the yellow star we call the sun, our planet moves to a rhythm, its poles tilting minutely one way then the other, first toward the light and heat of the sun, then away again. This rhythm ticks away time for us, time that we count as years. The rapidly ticking beat of our years is like a continuous purr against the background of other rhythms slower than we can comprehend.

At times, in response to these greater patterns, our planet has cooled and at others warmed. A few heartbeats ago in the time frame of our planet, a severe chilling phase brought ice sheets to large areas of the Northern Hemisphere, covering much of Europe, northwestern North America and parts of Northeast Asia. As the great ice sheets retreated, they left behind frigid land that had been scoured by ice for millennia. Where the ice retreated, life advanced. Plants and animals colonized what snow and ice vacated.

The part of East Asia that includes Honshu, Shikoku and Kyushu was spared the severe impact of the great glaciers. Montane glaciers descended some distance, but the worst of the impact was avoided. As a result, life from before the last Ice Age survives in Japan, a greater diversity of plants than elsewhere in temperate Asia.

Here the diversity of the temperate zone is at its greatest and most conspicuous, and the passing of the seasons is powerfully registered. It is by the undulating pulse of the seasons that we really know this region. As our northerly pole tips imperceptibly sunward, the short winter days lengthen toward spring, and almost before we are aware of the change, trees and shrubs are pulsing into color, budding flowers and leaves with astonishing speed.

The period of dormancy ends with a rush powered by sunlight as the fresh green tracery of leaves spreads to form a light-absorbing blanket, powering their further growth and survival.

As the flower and leaf buds swell so too do the sounds of the temperate zone. Resident birds steal a brief march on the migrants, singing and establishing territories before the migrants reach these shores. Then comes the shrilling, buzzing and whining of the insects. Once the volume of the cicadas has risen to the point where it drowns out the songbirds, summer can be said to have arrived.

This period of the year is a time of darkening greens, of storing energy. The pulse of the planet is already ticking down, the sunward tilt of the northern pole changing so that the days shorten. Plants and animals alike become aware of the passing season and, imperceptibly at first, respond, hurrying to rear their last young of the year.

As the fractional tilt of the Earth pulls the northern region slightly farther from our source of heat, plants prepare for the months ahead by withdrawing their investment in their leaves. They shunt the more valuable components, such as chlorophyll, back into their main stems; leaves blush, then rage into color.

Colors change day by day during the autumn, scents change as the loamy smell of fungi hangs in the air and the sounds change yet again. The last of the summer songs is heard and the insects engage in a last burst of activity -- the dry rustle of dragonfly wings replaces the buzzing whirr of cicadas. Winter, the season that dominates the year in the Arctic and subarctic, is less powerful here in the temperate region; yet it still defines the year, limits the life that can survive here.

Spring, summer and autumn are all easy to weather, but for species that thrive in summer's warmth, surviving the subzero temperatures of winter is the true gauntlet to run. Winter is a kind of bottleneck for species of the temperate zone. Anything uncertain of its survival chances has to flee and may not return for half a year or more. Anything not firmly rooted down is likely to be blown away or frosted off.

This is the season of darkness; nearly half of each day is unlighted, and many species just give in and switch down their regulators, leaving only their pilot lights on standby, waiting for the subtle tilt of the earth to bring favorable conditions once more.