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   39109篇
  免费   3518篇
  国内免费   21篇
  42648篇
  2023年   236篇
  2022年   494篇
  2021年   1061篇
  2020年   610篇
  2019年   773篇
  2018年   921篇
  2017年   772篇
  2016年   1200篇
  2015年   1997篇
  2014年   2225篇
  2013年   2366篇
  2012年   3327篇
  2011年   3034篇
  2010年   1963篇
  2009年   1689篇
  2008年   2409篇
  2007年   2361篇
  2006年   2135篇
  2005年   2017篇
  2004年   1855篇
  2003年   1740篇
  2002年   1617篇
  2001年   345篇
  2000年   241篇
  1999年   344篇
  1998年   422篇
  1997年   283篇
  1996年   250篇
  1995年   216篇
  1994年   217篇
  1993年   226篇
  1992年   216篇
  1991年   206篇
  1990年   189篇
  1989年   168篇
  1988年   180篇
  1987年   162篇
  1986年   129篇
  1985年   133篇
  1984年   169篇
  1983年   127篇
  1982年   169篇
  1981年   148篇
  1980年   134篇
  1979年   105篇
  1978年   109篇
  1977年   104篇
  1976年   85篇
  1975年   74篇
  1973年   83篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
11.
12.
13.
14.
The phenotypic consequences in the house mouse (Mus musculus domesticus) of the transition from an ordinary field-dwelling rodent to a species that is dependent on human populations was studied by investigating the morphometric variation of non-commensal populations of M. m. domesticus from Syria, Jordan, SW Iran, and Libya and comparing them with that of conspecific commensal populations from Eastern Turkey, Greece, and Bulgaria. Commensal populations of M. musculus musculus from the Czech Republic were used as an outgroup. About 849 adult specimens of M. musculus were analysed by multivariate procedures based on standard molar, skull and body measurements. As expected, there was considerable variation among the studied populations and a good correspondence between morphometric and geographic distances. The resulting morphometric tree was consistent with the hypothesis that the original radiation of M. m. domesticus took place somewhere in the Near East. Commensal populations of M. m. domesticus form a single derived branch. Specimens originating from four different sites in eastern Syria showed the greatest similarity to one another and possessed relatively bigger molars than the other studied populations. Commensal populations were characterised by longer tails when compared to non-commensal populations, which suggests an adaptation for living in a more three-dimensionally heterogeneous environment for commensal populations.  相似文献   
15.
16.
The economic viability of the wildlife based enterprises (bee-keeping and caterpillar utilization) in Malawi is discussed in relation to conventional agricultural enterprises (maize, beans and ground-nuts). A strong incentive emerges for rural people to adopt wildlife management as an adjunct to subsistence agriculture, and therefore, to promote conservation of natural ecosystems and wildlife habitats in the face of growing human population and demand for land. Dependence on agriculture has depleted the wildlife resource outside protected areas and has been less effective in improving the wealth and living standards of most rural people. This study illustrates that the Malawi Department of National Parks and Wildlife needs to introduce economic incentives that integrate biological conservation with economic development for the rural people. The management programme involves the adoption of a rotation burning policy that promotes vegetation coppicing, eases harvesting and promotes high caterpillar yields.  相似文献   
17.
Vocalizations of South African clawed frogs are produced by contractions of laryngeal muscles innervated by motor neurons of the caudal medulla (within cranial nerve nucleus IX-X). We have traced afferents to laryngeal motor neurons in male and female frogs using retrograde axonal transport of horseradish peroxidase conjugated to wheat germ agglutinin (HRP-WGA). After iontophoretic injection of HRP-WGA into n. IX-X, retrogradely labelled neurons were seen in the contralateral n. IX-X, in rhombencephalic reticular nuclei, and in the pre-trigeminal nucleus of the dorsal tegmental area (DTAM) of both males and females.  相似文献   
18.
19.
20.
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