首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4261篇
  免费   142篇
  2020年   62篇
  2019年   64篇
  2018年   109篇
  2017年   113篇
  2016年   137篇
  2015年   94篇
  2014年   96篇
  2013年   1217篇
  2012年   114篇
  2011年   144篇
  2010年   126篇
  2009年   98篇
  2008年   108篇
  2007年   78篇
  2006年   71篇
  2005年   70篇
  2004年   61篇
  2003年   30篇
  2002年   47篇
  2001年   64篇
  2000年   51篇
  1999年   62篇
  1998年   47篇
  1997年   45篇
  1996年   42篇
  1995年   31篇
  1994年   39篇
  1993年   43篇
  1992年   45篇
  1991年   36篇
  1990年   29篇
  1989年   37篇
  1988年   35篇
  1987年   45篇
  1986年   31篇
  1985年   49篇
  1984年   38篇
  1983年   65篇
  1982年   48篇
  1981年   56篇
  1980年   54篇
  1979年   38篇
  1978年   40篇
  1977年   58篇
  1976年   42篇
  1975年   44篇
  1974年   31篇
  1973年   30篇
  1972年   26篇
  1971年   33篇
排序方式: 共有4403条查询结果,搜索用时 15 毫秒
961.
Focus in this discussion is on research designed to examine human fertility variation in 59 Pennsylvania counties in 1850 and 1860. The research goes beyond previous historical studies in examining the relationship between land quality and fertility and in separating the possible impacts of settlement from the availability of land for agricultural purposes. The measure of human fertility used in the analysis was the child-woman ratio, defined as the number of children aged 0-4/1000 white women aged 15-49. In 1850 the child-woman ratio ranged from 488-889. The child-woman ratio fell slightly between 1850 and 1860, and county level variation was reduced. Yet, Pennsylvania counties varied substantially in fertility levels during this period. The range between the highest and lowest fertility countries was over 400 children in 1850 and almost 350 in 1860. The set of agricultural variables displayed expected differences at the 2 time periods. The excess demand for farmsites increased between 1850 and 1860, while the variation in demand decreased by over one-half, reflecting additional settlement within all acres of the state. The settlement ratio increased over the decade; the sex ratio declined, most likely in response to outmigration westward. The agricultural variables were all negatively related to the child-woman ratio and were statistically significant, except for the labor/acre variable. The strongest correlation in 1850 was between the excess demand for farmsites and fertility. This coefficient indicated that the greater the excess demand for farmsites, the lower the child-woman ratio. This relationship was attentuated somewhat for 1860, yet it continued to be negative and statistically significant. The socioeconomic and demographic variables were all related to fertility in the expected direction, but only 5 of the 7 correlations were statistically significant in 1850 and 6 in 1860. The settlement ratio, sex ratio, percent urban, distance to urban place, and the measure of female age composition were all significantly related to fertility in 1850 and 1860. The study results support the growing body of research which has identified agricultural opportunity as a significant factor in fertility in rural areas of 19th century America. The findings also suggest that the importance of agricultural opportunity extended beyond the frontier period.  相似文献   
962.
963.
964.
965.
The paper analyzes the limiting case for a modified Yule-Simon model of city growth. The aim of the paper is primarily to reintroduce geographers to the notion of a closed system of cities, i.e., a system of cities where the aggregate population is stationary in size. The Yule-Simon model, while successful in dealing with the observed properties of open systems, is seen to break down for closed systems. Moreover, incorporating Simon's assumptions regarding intercity migration into the model produces implausible results. Also, the traditional approach of employing entropy-maximizing procedures assumes a certain pattern of intercity migration. This pattern, while not inherently implausible, does present features which are questionable. The authors conclude that an adequate theory for closed systems has yet to be developed.  相似文献   
966.
Fired rock-crystal was found within the matrix of a Balangodese (pre-Vedda) burial in a Mesolithic midden in Ceylon; thermoluminescent measurements indicated a date of about 7000 years before present.  相似文献   
967.
968.
969.
970.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号