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   428765篇
  免费   48296篇
  国内免费   157篇
  477218篇
  2018年   3968篇
  2016年   5370篇
  2015年   7072篇
  2014年   8332篇
  2013年   11433篇
  2012年   13060篇
  2011年   13476篇
  2010年   9211篇
  2009年   8559篇
  2008年   12307篇
  2007年   12792篇
  2006年   12010篇
  2005年   11476篇
  2004年   11482篇
  2003年   10760篇
  2002年   10584篇
  2001年   17491篇
  2000年   17520篇
  1999年   13980篇
  1998年   5061篇
  1997年   5304篇
  1996年   4945篇
  1995年   4637篇
  1994年   4518篇
  1993年   4557篇
  1992年   11653篇
  1991年   11584篇
  1990年   11343篇
  1989年   10975篇
  1988年   10546篇
  1987年   10130篇
  1986年   9394篇
  1985年   9263篇
  1984年   7779篇
  1983年   6733篇
  1982年   5193篇
  1981年   4652篇
  1980年   4493篇
  1979年   7452篇
  1978年   5904篇
  1977年   5430篇
  1976年   5225篇
  1975年   5624篇
  1974年   6336篇
  1973年   6198篇
  1972年   5781篇
  1971年   5250篇
  1970年   4652篇
  1969年   4593篇
  1968年   4431篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
852.
Like many species, the Mormon Metalmark butterfly (Apodemia mormo) has been given conservation ranking in Canada based on limited data. This species is widespread across western North America, but has only two populations in Canada: an “endangered” population in the Similkameen valley of British Columbia, and a “threatened” population in Grasslands National Park (GNP) in Saskatchewan. Here we present genetic data from 1498 base pairs of the cytochrome oxidase I gene sequence and six novel microsatellite loci in order to assess (1) whether the two populations are related, (2) the degree to which they are genetically diverse and demographically stable, and (3) what their relationships are to the nearest unranked populations of A. mormo across the Canada-United States border. Our principal conclusion is that the two populations are not closely related genetically. We also found that the British Columbia population is genetically depauperate and, with the exception of the nearest neighboring populations across the border, not recently genetically connected to other populations in the Pacific Northwest. In comparison, the Saskatchewan population is genetically diverse, and gene flow occurs with several other eastern populations. Population structure was not detected within either the British Columbia or the Saskatchewan populations. This research supports the prior conservation rankings of both populations and provides new insight that will help to inform future management decisions for the Canadian populations of this charismatic butterfly.  相似文献   
853.
854.
855.
856.
The small-scale vertical distribution of five genera of holoplanktonic molluscs inhabiting the upper oceanic layer (0–105 m) was analysed to test whether evident distribution patterns could be attributed to habitat partitioning. Zooplankton samples were taken over neritic waters during the day and night using a 505-μm multiple closing net at five levels (0–6, 6–12, 12–18, 45–55 and 95–105 m) of the water column. Flowmeters were placed in each net to estimate the amount of filtered water. From the 55,654 identifiable specimens of holoplanktonic molluscs, only 6.2 % were in juvenile stage, and Cavolinia, Diacavolinia, Diacria, Oxygyrus and Clio were the most abundant genera. Adults of these genera comprised 3 % of the total abundance. The abundance data were examined using a spatial segregation index, and its significance was tested with null model methods based on Monte Carlo randomizations. Results indicated that adults showed a much more overlapped distribution in the water column than the in the earlier stages. Indeed, juveniles of Cavolinia, Diacavolinia and Oxygyrus were mainly found in the 0–18-m layer, whereas Diacria and Clio were recorded in the 45–105-m stratum. Null model arguments revealed a significant vertical segregation among them. Potential ecological factors involve preferential spawning areas of adults, avoidance of competition for feeding and spatial resources, use of visual capabilities in searching prey, brood protection and avoidance of strong turbulence conditions.  相似文献   
857.
858.
The study's objective is to survey the attitudes of Chinese people living in Hong Kong toward genomic science and technology (GST) and their ethical and social implications. Using a 24-item questionnaire, 877 Cantonese-speaking residents between age 18 and 64 with minimum high school education are interviewed by telephone. Multiple regression analysis identifies education level as the most important demographic variable. Overall, respondents have mild agreement with genetic determinism and the use of GST for disease prevention but not for non-therapeutic genetic enhancement and production of “genetically modified” crops or meat. Respondents strongly believe that GST tampers with nature and resources should be used to solve other healthcare problems first. Respondents also show little concern that personal genetic information may be abused by their employers or schools and have only a minimal willingness to share personal genetic information with their family members.  相似文献   
859.
860.
At present, the method of current drive by means of lower hybrid waves is not applied to low-aspect-ratio tokamaks, because, in the traditional approach, it would be necessary to use waves with a very high slowing-down factor. However, studies of new transparency regions for waves in a nonuniform magnetized plasma, performed earlier at the Ioffe Physical Technical Institute, Russian Academy of Sciences, made it possible to develop an approach in which slow waves are excited in the poloidal (rather than toroidal) direction. In this approach, moderately slowed-down waves first propagate in the poloidal direction, but then turn in the toroidalal direction and get into the dense plasma. In this work, this approach is further developed using numerical methods. In particular, the influence of the density profile in the edge plasma on the efficiency of wave excitation for given antenna parameters is studied in detail.  相似文献   
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