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   3711篇
  免费   90篇
  2020年   39篇
  2019年   52篇
  2018年   46篇
  2017年   70篇
  2016年   68篇
  2015年   53篇
  2014年   59篇
  2013年   1260篇
  2012年   61篇
  2011年   81篇
  2010年   66篇
  2009年   61篇
  2008年   59篇
  2007年   57篇
  2006年   47篇
  2005年   41篇
  2004年   45篇
  2003年   44篇
  2002年   33篇
  2001年   44篇
  2000年   52篇
  1999年   84篇
  1998年   56篇
  1997年   42篇
  1996年   41篇
  1995年   31篇
  1994年   38篇
  1993年   36篇
  1992年   31篇
  1990年   37篇
  1989年   36篇
  1988年   29篇
  1987年   43篇
  1986年   34篇
  1985年   43篇
  1984年   48篇
  1983年   57篇
  1982年   49篇
  1981年   35篇
  1980年   61篇
  1979年   44篇
  1978年   45篇
  1977年   52篇
  1976年   41篇
  1975年   49篇
  1974年   39篇
  1973年   26篇
  1970年   35篇
  1969年   27篇
  1967年   27篇
排序方式: 共有3801条查询结果,搜索用时 109 毫秒
101.
This essay synthesizes the history of the birth control movement in the US and describes changes in sexual behavior, social values, and public policy in order to provide a context for the changes in human reproductive public policy. After an introduction, the essay outlines the history of contraception from the early nineteenth to the early twentieth centuries. Part 3 covers the period of World War I to the Depression when civil libertarians and eugenicists began to question the suppression of contraception and Margaret Sanger organized her clinics. The fourth part of the essay carries the history forward to the end of World War II, a period in which Dr. Clarence J. Gamble began to expose the marketing of defective contraceptive methods and to illustrate the willingness of poor women to accept contraceptives. The social changes which began in the 1950s are the subject of the fifth section of the essay. During this period, Roman Catholic opposition to contraception lessened, and social scientists began to focus world attention on overpopulation. Frank Notestein was appointed the first head of the Office of Population Research at Princeton, and John D. Rockefeller III founded the Population Council which conducted research into the IUD and began to attempt to influence population growth in nonindustrialized countries. This period also saw the development of the oral contraceptive. The changes of this era were institutionalized in 1967 when the federal government took a positive stance towards family planning in its Social Security Amendments. The decade of the 1970s is the subject of the last part of this essay. This period saw the Supreme Court assign a constitutionally protected right to abortion and Congress pass the Helms Amendment which denied the use of foreign aid funds for abortions. Challenges to the right to individual birth control practice continued during this period, and debate centered around the specter of overpopulation, the threat of adolescent pregnancy, and perceptions of "family values."  相似文献   
102.
A trial, monitored release on the Wingecarribee River over 27 days had a peak discharge below the Wingecarribee Dam of 1090 ML/d, which had a flow duration of 0.95 per cent. Morphological impacts on 29 km of channel below the dam included bank erosion, concentrated neck overflow, bed degradation and channel widening at recent cutoffs, sedimentation in weir pools and overbank flow, particularly at discharges greater than 800 ML/d. Prolonged operational releases at bankfull discharge (400–800 ML/d) will have the greatest morphologic impacts and should be avoided. Pulses which dissipate stream power over a broad, well-vegetated floodplain should be incorporated in the operational release policy, provided such pulses are phased to facilitate channel and vegetation recovery.  相似文献   
103.
104.
105.
106.
107.
108.
The Early Historic Period in the Mun Valley occupies a critical three centuries, from A.D. 500 to A.D. 800, a period often known as that of Zhenla, a name derived from early Chinese accounts. This article first describes early findings of fieldwork designed to illuminate the prehistory of the upper Mun Valley, a period which covers approximately two millennia, with initial settlement now dated between 1500 and 1000 B.C. It then considers the nature of the transition to the complex polities of Zhenla. This involves a combination of archaeological and documentary evidence. Sources for the latter are rare for the Mun Valley, but by broadening the area considered, it is possible to integrate archaeological and historical sources in order to portray the intensifying social changes which characterize the Early Historic Period. It is suggested that most of the Mun Valley was occupied by Khmer speakers during the Iron Age, people responsible for the transition to increasingly centralized polities of the Early Historic Period. In the upper reaches of the Mun catchment, and in the Chi Valley to the north, the late prehistoric inhabitants spoke Mon. They had close affinities with the Dvaravati polities of Central Thailand. If confirmed, this hypothesis would indicate a series of local trends towards increasing social complexity which we can recognize in the early epigraphic record.  相似文献   
109.
This study examined landcover change during 1940–1988 in two contiguous landscapes of approximately 12,400 hectares in central Ohio, USA: an inertial till plain area and a more dynamic moraine area. Agriculture dominated both landscapes throughout, however, the cover of natural vegetation and urban-suburban development increased over time on the moraine while natural vegetation decreased and agriculture increased on the till plain. Markov process models for landcover change were constructed for three 14–17-year time intervals and for the entire forty-eight-year study period. Probabilities of self-replacement for agricultural lands (0.91–0.97) were higher and those for forests (0.54–0.65) lower than those reported for other landscapes. Predictions of landcover percentages for fifty years in the future were made using Markov process models derived from actual cover changes in 1940–1957, 1971–1988, and 1940–1988. All three models produced similar predictions for the inertial till plain area. In contrast, the three models' predictions differed considerably for the moraine area. Only the 1971–1988 transition matrix was sensitive to increases in urbanization and reforestation which began on the moraine in the 1970s. These results indicate that reliance on lengthy study intervals and net rates of change can obscure spatial and temporal patterns of landcover change that are caused by natural and socioeconomic factors operating on shorter time scales.  相似文献   
110.
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