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   35272篇
  免费   3668篇
  国内免费   20篇
  2021年   387篇
  2020年   318篇
  2019年   434篇
  2018年   493篇
  2017年   511篇
  2016年   705篇
  2015年   1169篇
  2014年   1212篇
  2013年   1690篇
  2012年   1787篇
  2011年   1741篇
  2010年   1377篇
  2009年   1131篇
  2008年   1409篇
  2007年   1460篇
  2006年   1402篇
  2005年   1436篇
  2004年   1508篇
  2003年   1491篇
  2002年   1438篇
  2001年   814篇
  2000年   785篇
  1999年   801篇
  1998年   531篇
  1997年   487篇
  1996年   484篇
  1995年   443篇
  1994年   372篇
  1993年   367篇
  1992年   720篇
  1991年   589篇
  1990年   616篇
  1989年   617篇
  1988年   497篇
  1987年   516篇
  1986年   456篇
  1985年   474篇
  1984年   505篇
  1983年   395篇
  1982年   390篇
  1981年   392篇
  1980年   330篇
  1979年   418篇
  1978年   354篇
  1977年   361篇
  1976年   296篇
  1975年   287篇
  1974年   315篇
  1973年   276篇
  1971年   220篇
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
131.
132.
The role of territoriality was investigated by studying 25 impala rams at a reserve in the Waterberg region of South Africa (23°45′S, 28°23′E). Mean territorial tenure was 67.25 days (range 23–99), with a mean territory size of 21.0 ± 11.27 ha, compared with home ranges of 34.1 ± 9.03 ha for territorial rams and 58.8 ± 33.35 ha for bachelor rams, using the fixed kernel method. Territory boundaries remained constant, whilst the area surrounding important features such as water holes, appears to be neutral in terms of territoriality. The rut, as evidenced from peaks in chasing and roaring, lasted for 2 months from 10 April to 10 June 2001, with intensified behaviour including matings observed from 16 May to 4 June 2001. Territorial rams chase and roar more than bachelors. Flehmen and display behaviours are performed by all rams, whilst fights and other reproductive behaviours are generally rare. Bachelors browse more than territorial rams. Only bachelors spar and allogroom, and orally groom themselves more than territorial rams.  相似文献   
133.
134.
Graphite furnace atomic absorption spectrometry, after improved matrix modification and using Zeeman background correction, was used to measure the serum selenium content of healthy adults living in the Antwerp region (Belgium). The mean serum concentration of 13 men and 13 women, sampled once a month during 1 year, was 84.3 +/- 9.4ng/ml with a broad range of 51.4-121.7 ng/ml. The intra-individual variation was remarkably high. Recent literature on selenium concentrations is reviewed and values are tabulated, with limitation to healthy adults and European countries. The mean serum selenium concentration measured corresponded well to older literature data for Belgium. The obtained values were found to be in the medium range compared with the literature data for other European countries.  相似文献   
135.
A heptageniid mayfly species Paegniodes dao sp. nov. (Ephemeroptera: Heptageniidae) is described from Vietnam. The larva is distinguished by round anterolateral margins of pronotum, triangular‐shaped labrum, rudimentary lamella and well developed fibrillae of gills 1, and light brown body that lacks distinct markings. Diagnosis, line drawings, material, and habitat data are provided.  相似文献   
136.
137.
138.
Healing the Modern in. Central Javanese City. Steve Ferzacca. Durham, NC: Carolina Academic Press, 2001. 280 pp.  相似文献   
139.
140.
Until recently, the most common parametric approaches to study the combined effects of several genetic polymorphisms located within a gene or in a small genomic region are, at the genotype level, logistic regressions and at the haplotype level, haplotype analyses. An alternative modeling approach, based on the case/control principle, is to regard exposures (e.g., genetic data such as derived from Single Nucleotide Polymorphisms – SNPs) as random and disease status as fixed and to use a marginal multivariate model that accounts for inter‐relationships between exposures. One such model is the multivariate Dale model. This model is based on multiple logistic regressions. That is why the model, applied in a case/control setting, leads to straightforward interpretations that are similar to those drawn in a classical logistic modeling framework. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  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