Natural selection is the process by which one type of animal within a species thrives because of certain characteristics that make it more likely to live than others in its group. The history of the peppered moth(灰蛾) is an example of the natural selection process.

In 19th century England, certain types of peppered moths were able to better blend（融合）into their surroundings. During that time period, great changes were happening in Great Britain. The Industrial Revolution was part of this change, and with it came air pollution. Natural selection often takes hundred or even thousands of years to occur. For the peppered moth, this process occurred comparatively quickly.

At the beginning of the Industrial Age, most peppered moths in England were light-colored and covered with black markings, although a few moths had dark-colored wings. Because the light-colored moths blended into the light-colored bark on the trees, they could not be easily seen by birds that would eat them. As the air grew more polluted, however, tree trunks became covered with soot(烟煤) and became darker. The light-colored moths became easy for birds to see against the dark tree trunks. Since the dark-colored moths now had the advantage, their numbers grew. Within 50 years, the peppered moth went from being mostly light-colored to being mostly dark-colored.

In the 20th century, the air cleared up, and the peppered moth population changed again. As tree trunks lighted due to less soot in the air, light-colored moths once again had an advantage. Their numbers increased as soot levels declined. Depending on their environment, the coloration of the moths helped them to be “naturally selected” to survive.

The flying fox is not a fox at all. It is an extra large bat that has got a fox’s head, and that feeds on fruit instead of insects. Like all bats, flying foxes hang themselves by their toes when at rest, and travel in great crowds when out flying. A group will live in one spot for years. Sometimes several hundred of them occupy a single tree. As they return to the tree toward sunrise.

Flying foxes have babies once a year, giving birth to only one at a time. At first the mother has to carry the baby on her breast wherever she goes. Later she leaves it hanging up, and brings back food for it to eat. Sometimes a baby bat falls down to the ground and squeaks for help. Then the older ones try to pick it up. If they fail to do so, it will die. Often hundreds of dead baby bats can be found lying on the ground at the foot of a tree.

Aviculturists, people who raise birds for commercial sale, have not yet learned how to create similar conditions to the natural incubation (孵化) of parrot eggs in the wild. They continue to look for better ways to increase egg production and to improve chick survival rates.

When parrots incubate their eggs in the wild, the temperature and humidity (湿度) of the nest are controlled naturally. Heat is transferred from the bird’s skin to the top of the eggshell, leaving the sides and bottom of the egg at a cooler temperature. This temperature difference may be vital to successful hatching. Nest construction can contribute to this temperature difference. Nests of loosely arranged sticks, rocks, or dirt are cooler in temperature at the bottom where the egg contacts the nesting material. Such nests also act as humidity regulators by allowing rain to drain into the bottom sections of the nest so that the eggs are not in direct contact with the water. As the water that collects in the bottom of the nest changes into gas, the water vapor rises and is heated by the incubating bird, which adds significant humidity to the incubation environment.

In artificial incubation programs, aviculturists remove eggs from the nests of parrots and incubate them under laboratory conditions. Most commercial incubators heat the eggs fairly evenly from top to bottom, thus ignoring the bird’s method of natural incubation, and perhaps reducing the survivability of the hatching chicks.

When incubators are not used, aviculturists sometimes suspend wooden boxes outdoors to use as nests in which to place eggs. In areas where weather can become cold after eggs are laid, it is very important to maintain a deep foundation of nesting material to protect eggs against the cold bottom of the box. If eggs rest against the wooden bottom in extremely cold weather conditions, they can become chilled to a point where the embryo (胚胎) can no longer survive Similarly, these boxes should be protected from direct sunlight to avoid high temperatures that are also fatal to the growing embryo.

Nesting material should be added in sufficient amounts to avoid both extreme temperature situations mentioned above and assure that the eggs have a soft, secure place to rest.