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   5139篇
  免费   205篇
  2020年   69篇
  2019年   75篇
  2018年   104篇
  2017年   126篇
  2016年   137篇
  2015年   102篇
  2014年   71篇
  2013年   1440篇
  2012年   97篇
  2011年   112篇
  2010年   134篇
  2009年   132篇
  2008年   134篇
  2007年   108篇
  2006年   96篇
  2005年   76篇
  2004年   67篇
  2003年   69篇
  2002年   70篇
  2001年   62篇
  2000年   73篇
  1999年   84篇
  1998年   67篇
  1997年   51篇
  1996年   58篇
  1995年   57篇
  1994年   54篇
  1993年   59篇
  1992年   55篇
  1991年   36篇
  1990年   36篇
  1989年   57篇
  1988年   48篇
  1987年   56篇
  1986年   54篇
  1985年   77篇
  1984年   96篇
  1983年   86篇
  1982年   65篇
  1981年   73篇
  1980年   77篇
  1979年   71篇
  1978年   72篇
  1977年   56篇
  1976年   75篇
  1975年   77篇
  1974年   49篇
  1973年   39篇
  1972年   37篇
  1970年   31篇
排序方式: 共有5344条查询结果,搜索用时 8 毫秒
1.
2.
Evidence from several disciplines supports the proposition that following the Wisconsinan and an initial but brief moist phase of the Holocene, there was a gradual drying trend in the mid-continent Prairie Peninsula that peaked about 7000 years ago, a pattern that was not reversed until about 4000 years ago. Except for minor perturbations, conditions during the past four millennia have essentially been similar to those of the present. Studies in palynology and geomorphology have shown that the climatic regimes of the early/mid Post-glacial had a marked influence on both the biota and landscape erosional patterns, especially along the margins of the Prairie Peninsula. Measurable changes in the landscape and attendant biota show a time-transgressive pattern of drought related phenomena recorded palaeo-ecologically from west to east.Few studies of archaeological sites in the Prairie Peninsula have examined the evidence in and around the sites themselves for data reflecting local environmental dynamics. In fact, earlier models proposed for explaining the evolution of food-procuring systems in eastern North America virtually ignored potential changes in the natural environment. This paper examines data from two archaeological sites along the prairie-forest border in Missouri that contain long cultural sequences. Evidence is presented that pronounced changes in the natural environment occurred and that these biophysical variables are indeed critical for understanding the evolution of food-procuring societies in the American Midwest.  相似文献   
3.
4.
5.
6.
7.
Pearls have been a valued resource in most cultures that had access to them. A number of historically important pearling grounds were situated in the waters around today's Indonesia. One of these areas, now largely forgotten, was the Segara Anakan lagoon in South Java. In the seventeenth century, Dutch colonists exploited the lagoon's pearls. Afterwards, the lagoon's oysters were locally exploited as a food item until the late 1970s. While the pearl fishery attracted considerable attention in the colonial literature, its disappearance, by contrast, went largely undocumented. Nowadays, the oysters no longer are found in the lagoon as a result of extensive sedimentation processes. Their former existence is only preserved in the memory of local people. This article examines the history and fate of the pearls of Segara Anakan, providing an example of a formerly valued species whose existence simply became forgotten outside the area.  相似文献   
8.
Trigger levels for fine suspended sediment (FSS) load in streams are required to identify rainfall-runoff events that have significantly elevated FSS loads, compared with expected background loads, as a result of catchment disturbance. Stream FSS load data collected in the mine-impacted Magela Creek catchment in the wet-dry tropics were used to derive water quality management trigger levels for two approaches – a Before-After-Control-Impact Paired difference design (BACIP) and a regression relationship between observed FSS load and corresponding event discharge characteristics. The results indicate that both the BACIP and regression relationship approaches behave similarly, with similar FSS load events elevated above trigger levels. Notwithstanding this, it is recommended that in order to reliably assess the location and magnitude of a catchment disturbance on FSS load, a combination of BACIP and regression relationship approaches must be adopted. In this study, an event with a FSS load above trigger levels associated with both BACIP and the regression relationship fitted for the downstream site is considered to be significantly elevated as a result of a disturbance within the mine-impacted region. While this technique cannot conclusively determine whether the cause of the disturbance is mine-related or natural (such as fire or bank erosion), it is an efficient statistical method of identifying events that warrant further investigation and management action, if required.  相似文献   
9.
This study explores the spatial structure of retailing in Milwaukee, Wisconsin, in the era before the introduction of the electric streetcar in 1890. It uses city directory listings to chart changes in the numbers and types of retail outlets, their degree of nucleation and the functional complexity of such nucleations. The evidence suggests the early appearance of non-central retailing, the rapid dispersal of high as well as low order goods to non-central sites and the early emergence initially of a three-level and then, by 1870, a four-level hierarchy of shopping clusters. The pattern of retail clustering varied with the class and ethnic character of neighbourhoods, and clusters gravitated toward major arterial routes even in the pre-mass transit ear. Urban retailing in the second half of the nineteenth century was neither randomly dispersed nor over-whelmingly concentrated in a single central retail district, but developed a spatial hierarchy that clearly anticipated modern patterns.  相似文献   
10.
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