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.

Reasons to visit St.Louis

Whether you're traveling alone, with a friend or with the whole family, there's a nearly endless selection of activities you'll want to experience—so many that it's hard to decide where to go first.

Missouri Botanical Garden

    You could easily spend an entire day making your way through the 79 acres(英亩)on display here. The gardens also boast(以拥有……而自豪) seasonal exhibits, such as the summer Lantern Festival—taking place through July—that showcases beautiful works of light each year.

4344 Shaw Blvd, 314-577-5100, missouribotanicalgarden. org.

St. Louis Art Museum Free

    There are more than 33,000 works on display here, including the recent addition of 225 donated pieces of feature work by American artists and more than 200 pieces from Asia. The museum is free, and special exhibits are free on Fridays.

    I Fine Arts Drive, 314-721-0072, slam. Org
St. Louis Science Center Free

    With a wealth of displays to keep kids and adults alike entertained and learning all day, the center boasts an impressive 700 hands-on exhibits, including a fossil dig site, water tables in the discovery room, electrical energy demonstrations, and cyber activities.

    5050 Oakland Ave, 314-289-4400, slsc.org

St. Louis Zoo Free (Free for some attractions)

    The zoo includes beautiful naturalistic displays and boasts plenty of activities to keep kids excited, from feeding exhibitions and safaris(游猎), to sea lion shows and conservation talks.

    One Government Drive, 314-781-0900, stlzoo.org.

    Although his 1-year-old smart-phone still works perfectly, Li Jijia already feels the need to replace it.

    "There are many better ones available now. It's time to upgrade(更新）my phone."

    Li's impatience is shared by many. Shortly after the season when new products are released（发布，发售）, many consumers feel the urge to upgrade their electronic equipment, even though the ones they have still work just fine.

    As consumers' minds are occupied by Apple's newly released products and debate whether the Google tablet is better than the new Amazon Kindle, it might be time to take a step back and ask: "Do we really need the latest upgrades?"

    According to Donald Norman, an American author, "planned obsolescence (淘汰）" is the trick behind the upgrading culture of today's consumer electronics industry.

    Electronics producers strategically release new upgrades periodically, both for hardware and software, so that customers on every level feel the need to buy the newest version.

"This is an old-time trick—they're not inventing anything new," Norman said. "This is a wasteful system through which companies--many of them producing personal electronics—— release poor-quality products simply because they know that, in six months or a year, they'll put out a new one."

    But the new psychology of consumers is part of this system, as Norman admitted, "We now want something new, something pretty, the next shiny thing." In its most recent year, Apple's profit margin(利润) was more than 21 percent. At Hewlett-Packard, the world's biggest PC maker, it was only 7 percent.

    Apple's annual upgrades of its products create sales of millions of units as owners of one year's MacBook or iPhone line up to buy the newest version, even when the changes are slight.

    As to Li Jijia, the need for upgrading his smart-phone comes mainly from friends and classmates. When they are switching to the latest equipment, he worries about feeling left out.

    "Some games require better hardware to run," said Li. "If you don't join in, you lose part of the connection to your friends."

    When it comes to bringing up children, everybody agrees that it is not an easy task. Every parent watches eagerly the child's acquisition (学会) of each new skill — the first spoken words, the first independent steps, or the beginning of reading and writing. It is often tempting (吸引人的) to hurry the child beyond his natural learning speed, but this can set up dangerous feelings of failure and states of worry in the child. This might happen at any stage. A baby might be forced to learn something too early. A young child might be encouraged to learn to read before he knows the meaning of the words he reads. On the other hand, though, if a child is left alone too much, or without any learning chances, he loses his natural enthusiasm for life and his desire to find out new things for himself.

    Parents vary greatly in their degree of strictness towards their children. Some may be especially strict in money matters. Others are strict with the time of coming home at night or being on time for meals. In general, the controls show the needs of the parents and values (价值观) of society as much as the children's own happiness.

    When it comes to the development of moral standards (道德标准) in the growing child, consistency (前后一致性) is very important in parental teaching. To say "No" to a thing one day and excuse it the next is no foundation for morality (道德). Also, parents should realize that "example is better than precept". If they are not sincere and do not practice what they preach (说教), their children may grow confused and emotionally insecure when they grow old enough to think for themselves, and realize they have been to some extent cheated.

    A sudden realization of a big difference between their parents' principles and their morals can be a dangerous disappointment.

    Thirteen-year-old Kaylee has a lot of friends—532, actually, if you count up both her Instagram followers and Facebook friends. And she spends a lot of time with them.

    But is it possible that Kaylee's online friendships could be making her lonely? That's what some experts believe Connecting online is a great way to stay in touch, they say. However, some experts worry that many kids are so busy connecting online that they might be missing out on true friendships.

    Could this be true? During your parents' childhoods, connecting with friends usually means spending time with them in the flesh. Kids played Scrabble around a table, not Words With Friends on their phones. When friends missed each other, they picked up the telephone. Friends might even write letters to each other.

    Today, most communication takes place online. A typical teen sends 2,000 texts a month and spends more than 44 hours per week in front of a screen. Much of his time is spent on social media platforms like Instagram and Facebook.

    In fact, in many ways, online communication can make friend ships stronger, “There's definitely positive influence. Kids can stay in constant contact, which means they can share more of their feelings with each other,” says Katie, co-author of the App Generation.

    Other experts, however, warn that too much online communication can get in the way of forming deep friendships. “If we are constantly checking in with our virtual worlds, we will have little time for our real world friendships.” says Rosen, a professor at California State University.

    Rosen also worries that today s kids might mistake the “friends” on the social media for true friends in life. However, in tough times, you didn't need someone to like your picture or share your tweet. You needed someone who would keep your secrets and hold your hand. You would like to talk face to face.

    When it comes to medical care, many patients and doctors believe "more is better." But what they do not realize is that overtreatment—too many scans, too many blood tests, too many procedures—may bring harm. Sometimes a test leads you down a path to more and more testing, some of which may be invasive, or to treatment for things that should be left alone.

    Terrence Power, for example, complained that after his wife learned she had Wegener's disease, an uncommon disorder of the immune system, they found it difficult to refuse testing recommended by her doctor. The doctor insisted on office visits every three weeks, even when she was feeling well. He frequently ordered blood tests and X－rays, and repeatedly referred her to specialists for even minor complaints. Even when tests came back negative, more were ordered, and she was hospitalized as prevention when she developed a cold. She had as many as 25 doctor visits during one six－month period. The couple was spending about $30,000 a year for her care.

    After several years of physical suffering and near financial ruin from the medical costs, the couple began questioning the treatment after discussing with other patients in online support groups. "It's a really hard thing to determine when they've crossed the line," Mr. Power said. "You think she's getting the best care in the world, but after a while you start to wonder: What is the purpose?" Mr. Power then spoke with his own primary care doctor, who advised him to find a new specialist to oversee Mrs. Power's care. Under the new doctor's care, the regular testing stopped and Mrs. Power's condition stabilized. Now she sees the doctor only four or five times a year.