To understand the current situation, which is characterized by rapid increases in population, it is necessary to understand the history of population trends. Rapid growth is a comparatively recent phenomenon. Looking back at the 8,000 years of demographic history, we find that populations have been virtually stable or growing very slightly for most of human history. For most of our ancestors, life was hard, often nasty, and very short. There was high fertility (生育) in most places, but this was usually balanced by high mortality (死亡率). For most of human history, it was seldom the case that one in ten persons would live past forty, while infancy and childhood were especially risky periods. Often, societies were in clear danger of extinction because death rates could exceed their birthrates. Thus, the population problem throughout most of history was how to prevent extinction of the human race.

　　This pattern is important to notice. Not only does it put the current problems of demographic growth into a historical perspective, but it suggests that the cause of rapid increase in population in recent years is not a sudden enthusiasm for more children, but an improvement in the conditions that traditionally have caused high mortality.

　　Demographic history can be divided into two major periods: a time of long, slow growth which extended from about 8000 BC. till approximately 1650 AD. In the first period of some 9,600 years, the population increased from some 8 million to 500 million in 1650. Between 1650 and 1975, the population has increased from 500 million to more than 4 billion. And the population reached 6.2 billion throughout the world by the year 2000.One way to appreciate this dramatic difference in such abstract numbers is to reduce the time frame to something that is more manageable. Between 8000 BC. and 1650, an average of only 50,000 persons was being added annually to the world's population. At present， this number is added every six hours. The increase is about 800,000,000 persons annually.

【2017届高考英语一轮复习演练：1.3《My First Ride on a Train》（外研版含解析）】相关文章：

★ 【课堂新坐标】2017届高考英语（通用版）二轮复习书面表达专题训练：训练3 书信、邮件＋议论文（Ⅰ）（含解析）

★ 【三维设计】2017届高考英语（江苏专用）二轮复习课件：必修3 Unit 1《The world of our senses》

★ 【课堂新坐标】2017届高考英语（通用版）二轮复习短文改错专题训练：模式2 多一词（含解析）

★ 【课堂新坐标】2017届高考英语（通用版）二轮复习短文改错专题训练：训练2 短文改错（Ⅱ）（含解析）

★ 2014届高考英语一轮复习课时作业(十五)必修3Module 3《The Violence of Nature》（外研版）

★ 【三维设计】2017版（江苏专用）二轮复习练习：必修3 Unit 1《The world of our senses》课时跟踪检测B卷（含解析）

★ 2014届高考英语一轮复习课时作业(三)必修1Module 3《My First Ride on a Train》（外研版）

★ 【北师大版】2014届高考英语一轮复习指导课时作业：Unit 10 B卷 Word版含解析

★ （全国通用）2014届高考英语一轮单元复习 限时强化训练一

★ 2017届高考英语二轮复习书面表达限时测验：6（含解析）