Invasive species (入侵物种) are plants and animals which arrive in an area where they are not native, usually due to human activity. They are often able to grow quickly in their new homes because they have no natural enemies. As a result, they may replace or damage native plants and animals which live in the same environment.

Red squirrels used to be a common sight in Britain. Then, in the 1870s, grey squirrels were introduced from North America because rich people thought they looked fashionable in their large gardens. At the first sight, the two species of squirrels are similar. They both have a long tail, large eyes and small ears. However, the red squirrel has a length of around 19 to 23 cm and a body weight of 250 to 340 grams. Grey squirrels are larger. Their length measures between 23 and 30 cm and their body weighs between 400 and 600 grams. That allows them to store more fat and helps them to survive (生存) hard winters, making it possible for them to win the battle (战役) between the two species finally.

Three more differences explain why red squirrels have lost out in the competition with grey squirrels. First, red squirrels live high up in the trees, while grey squirrels spend more time on the ground. Any loss of forest habitat (栖息地) greatly affects red squirrels. Another reason is that grey squirrels can get used to a new environment more easily. For example, they are able to use food provided by humans. What’s worse, both squirrels carry Para poxvirus (副痘病毒), which doesn’t affect grey squirrels but is deadly to the reds.

It seems that scientists can’t do much to help red squirrels survive in Britain. Some politicians support destroying populations of grey squirrels, but many British people say that is cruel. Red squirrels have been successfully reintroduced from other countries.

They are now protected in places where there are no grey squirrels.

If you have brothers or sisters, the order in which you were born may make a big difference to choosing your job. A group of British scientists studied 500 people from 11 different career groups.

They found that elder children are more likely to become astronauts and scientists. NASA astronaut Neil Armstrong is the eldest child in his family. That’s probably because parents usually treat elder children more strictly than younger children. This helps them form a more disciplined (遵守纪律的)personality.

The scientists also found that middle children are 30 percent more likely to become CEOs. The scientists said that this could be because middle children need to do more to get attention from their parents. This makes them more flexible. Bill Gates, the former CEO of Microsoft, is the middle child.

Younger children, on the other hand, get attention by learning to pay attention to other people’s thoughts. This makes them sensitive (善解人意的) to others’ feelings. Therefore, younger children are more likely to become musicians. German musician Johann Sebastian Bach and Austrian composer Wolfgang Amadeus Mozart are both the youngest children in their families.

The study results also suggested that an only child is more likely to become an artist. He or she gets full attention from his or her parents and is freer to develop himself or herself. This makes him or her more creative.

Long ago, people didn’t need a lot of numbers, but when small villages grew into big cities, people started to need bigger numbers to count bags of crops and make trades. How did people in ancient time count numbers?

Paclfic Islands Some Pacific Island cultures count using the whole body. Different body parts stood for a different number, and the name of that part also means that number.	Rome Greek, Roman, and Hebrew mathematicians used letters for their numbers, The Romans, for example, used I for 1, V for 5, X for 10, L for 50, C for 100, and M for 1,000. To write 49, it took nine letters: XXXXVIIII.		Peru In South America, the Inca came up with a different way to note down numbers — by making knots on ropes. Different kinds of knots stood for different numbers, while the color of the rope may have showed what was being counted.
Maya Math The Maya of Central America (200-900 CE) were great mathematicians. They counted in groups of 20. They wrote their numbers from bottom to top, with the bottom row standing for one time (1s), the next row up for 20 times (20s), then 400 times (400s), and so on. They used bean-shaped dots● for 1, twig-shaped lines for 5, and they even had a shell shaped symbol for 0!

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