To many of us, the bilateral symmetry (左右对称) that characterizes the human face and the animal kingdom may be one of the first impressions we get of the world. If you reflect the left half of the face or body in a mirror, you obtain something that closely     1     the right half. Bilateral symmetry is so common that it could     2     be due to chance. If you think of the human body as a collection of some ten thousand trillion molecules, then there are countless ways to combine them into non-symmetrical shapes than into symmetrical ones. Why is it, then, that bilateral symmetry is so common? Given that all life on Earth slowly     3     through natural selection, the fact that we and many animals possess left-right symmetry, not, say, up-down or front-back symmetry, must have somehow     4     certain advantages. But how?

We can understand at least some parts of this natural “engineering”     5     by considering some simple physical principles. First, on the surface of the Earth, the Earth’s gravity introduces a     6     difference between up and down. Animals had to be able to remain stable and to cope with their weight under Earth’s perpetual downward pull. The ability to move further enhanced the difference between top and bottom. On one hand, birds had to handle the aerodynamics of flying, and on the other, be able to handle landing. Land animals had to develop some “mechanical” appendages (in other words, legs) to allow them to     7     themselves efficiently.

Animal locomotion was probably also     8     for the front-back asymmetry. Whether it was a bird, or a fish, having the sensory organs and the mouth in the front could mean the difference between finding lunch or missing it. The digestive and reproductive systems were pushed to the rear. Having eyes, ears, and noses in the front was also     9     for the avoidance of predators, and for the ability to explore an area before actually entering it. Life     10     that are fixed in one place or that move very slowly, such as trees or jellyfish, tend to have a different kind of symmetry, similar to that of a cone (球果) — they produce similar reflections in any mirror passing through their central axis.

An Encounter with Nature

As a nature photographer, I have to     1     the elements. But despite the wind and the rain, I still enjoy     2    outside in the wild. One of the best things about this job is that you can observe animals in their natural environment.

My favourite place to take photos is Yellowstone National Park. As the world’s first national park, Yellowstone is famous for the variety of its wildlife, but it is     3     best known for its bears. These huge, strong animals can live for 30 years. Despite a weight of up to 300 kilograms, they can run at a speed of around 64 kilometres per hour and are also     4     swimmers.

Last spring in Yellowstone, I followed a path that took me through a dark forest. When I finally stepped out of the trees, the view was     5    . An eagle flew over the snow-capped mountains, which were reflected in the still lake below. While I was concentrating on photographing this amazing scene, I     6     had a feeling that I was being watched. Slowly, and with the camera still held to my eye, I turned… and froze. Only meters away from me was a bear. With water     7     off its thick, brown hair, the bear stared back at me. Time stood still as the bear and I both waited to see who would move first. My legs started     8    . Somehow, I forced my finger to press the button. A second later, the bear turned and ran back into the forest. When I recovered from the shock, I looked at my camera. My most frightening but magical experience was now     9     forever in a single image.

From time to time I look at the photo as a     10     to show respect to all animals. It is after all we who are the visitors to their world.