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   969篇
  免费   51篇
  国内免费   83篇
  1103篇
  2024年   4篇
  2023年   15篇
  2022年   16篇
  2021年   16篇
  2020年   19篇
  2019年   27篇
  2018年   24篇
  2017年   24篇
  2016年   27篇
  2015年   24篇
  2014年   36篇
  2013年   68篇
  2012年   21篇
  2011年   28篇
  2010年   33篇
  2009年   37篇
  2008年   51篇
  2007年   53篇
  2006年   56篇
  2005年   58篇
  2004年   45篇
  2003年   43篇
  2002年   36篇
  2001年   29篇
  2000年   20篇
  1999年   35篇
  1998年   26篇
  1997年   22篇
  1996年   22篇
  1995年   26篇
  1994年   27篇
  1993年   21篇
  1992年   15篇
  1991年   14篇
  1990年   12篇
  1989年   9篇
  1988年   6篇
  1987年   6篇
  1986年   11篇
  1985年   6篇
  1984年   4篇
  1983年   2篇
  1982年   5篇
  1981年   3篇
  1980年   6篇
  1979年   3篇
  1978年   2篇
  1977年   4篇
  1976年   2篇
  1975年   2篇
排序方式: 共有1103条查询结果,搜索用时 11 毫秒
71.
应用快速酶联免疫法(ELISA)及Northern印迹杂交法研究了博莱霉素(BLM)同系物诱导癌基因表达的作用.通过检测P21和c-myc蛋白表达的改变和药物在RNA的转录水平上对癌基因表达的影响,证明BLM能够抑制c-myc基因的表达.这种抑制作用不仅发生在蛋白质的翻译水平,而且可能发生在RNA的转录水平上.BLMA6及A2对Ras基因亦有极显著的抑制,提示其亦为以p21蛋白为靶点的抗癌抗生素.A6、A2与A5之间的区别提示在同系物之间可能存在不同的抗癌机理  相似文献   
72.
Animal reintroduction and rewilding are two widely appealing and frequently connected forms of ecological restoration. However, the critical assumption that animal reintroduction automatically helps to restore formerly wild places is under‐theorized. To fill this void, we identified three common rewilding elements from the literature—ecological functioning, wilderness experience, and natural autonomy—and screened these against a hypothetical wolf reintroduction to Scotland. Each of the rewilding elements was likely to be positively impacted by a wolf reintroduction. Yet, there is a key conceptual difficulty in that the different rewilding elements do not necessarily enforce each other, and at times may even collide. Thus, a reintroduced species like the wolf may obfuscate the clear‐cut, purified nature category to which rewilding often aspires. As a way forward, we suggest that there is merit in actively engaging with the tensions created by rewilding and reintroductions. A reconceptualisation of the nature–culture spectrum as consisting of multiple layers (e.g. ecological functioning, wilderness experience, and natural autonomy) may help to interpret ecological restoration as a tentative, deliberative, and gradual enterprise. This bears some resemblance to the notion of approaching a landscape like a ‘palimpsest’ (i.e. a text built up of different layers written on top of each other), which may support the reconciliation of conflicting views without necessarily making those disappear. When viewed as feeding into a multilayered nature–culture spectrum, animal reintroduction and rewilding can be promoted as inspiring and essentially non‐controlling forms of ecological restoration and human interaction with nature.  相似文献   
73.
陕北黄土高原蒿属植物的分类与分析   总被引:1,自引:0,他引:1  
陕北黄土高原有蒿属植物30种1变种,居该地产种子植物属中所含种数的首位。所产蒿属植物在不同的植被带中梯度变化明显,替代现象显著。在生态类型上,旱生类型从南向北递增,中生类型从南向北递减。在区系组成上可分为6种分布区类型,即:我国特有分布,3种;温带亚洲分布,14种1变种;北温带及中亚分布各4种;旧世界温带分布,3种;东亚分布的2种。可见陕北黄土高原蒿属植物种类丰富,梯度变化明显,旱化现象显著,地理成分复杂,但以温带亚洲分布类型为主,兼有其它成分,属典型的温带性质。  相似文献   
74.
75.
Many food webs are affected by bottom‐up nutrient addition, as additional biomass or productivity at a given trophic level can support more consumers. In turn, when prey are abundant, predators may converge on the same diets rather than partitioning food resources. Here, we examine the diets and habitat use of predatory and omnivorous birds in response to biosolids amendment of northern grasslands used as grazing range for cattle in British Columbia, Canada. From an ecosystem management perspective, we test whether dietary convergence occurred and whether birds preferentially used the pastures with biosolids. Biosolids treatments increased Orthoptera densities and our work occurred during a vole (Microtus spp.) population peak, so both types of prey were abundant. American Kestrels (Falco sparverius) consumed both small mammals and Orthoptera. Short‐eared Owls (Asio flammeus) and Long‐eared owls (Asio otus) primarily ate voles (>97% of biomass consumed) as did Northern Harriers (Circus hudsonius, 88% vole biomass). Despite high dietary overlap, these species had minimal spatial overlap, and Short‐eared Owls strongly preferred pastures amended with biosolids. Common Ravens (Corvus corax), Black‐billed Magpies (Pica hudsonia), and American Crows (Corvus brachyrhynchos) consumed Orthoptera, Coleoptera, vegetation, and only a few small mammals; crows avoided pastures with biosolids. Thus, when both insect and mammalian prey were abundant, corvids maintained omnivorous diets, whereas owls and Harriers specialized on voles. Spatial patterns were more complex, as birds were likely responding to prey abundance, vegetation structure, and other birds in this consumer guild.  相似文献   
76.
At a time when all the political parties of Scotland are trying to establish a persuasive vision of the nation, inquiry into where ethnic and racial minorities fit into these debates provides one understudied means of bridging literatures on multinationalism and multiculturalism. Focusing especially on the lesser known question of how elite political actors are positioning minorities within projects of nation-building, this article draws upon original empirical data in which three predominant clusters emerge. The first centres on an aspirational pluralism, in so far as political elites are less inclined – in contrast to counterparts in some other minority nations – to place ethnically determined barriers on membership of Scottish nationhood. The second concerns the competing ways in which the legacy of Scotland's place in the British Empire is appropriated by actors of different political hues, and so assumes a multiform role. The third cluster points to potential limitations in minority claims-making and recognition, especially in terms of formal multilingualism and corporate multifaithism, something that may partly be explained by the tension between multinationalism and multiculturalism. Taken together, the article illustrates how elite political actors can play a vital role in ensuring that appeals to nationhood in Scotland can be meaningfully calibrated to include minorities too.  相似文献   
77.
In the interests of increasing fish production globally, alleviating global poverty, and income generation, marine aquaculture has seen worldwide expansion over the past few decades. However, this expansion has been matched by growing concerns over its environmental impacts. This article utilizes comparative legal analysis to explore the legal systems of governance for the environmental performance of marine aquaculture in four case study countries. Two are members of the European Community (Scotland, as part of the United Kingdom, and Greece), one is a recent accession country (Slovenia), and the fourth is closely linked to the European Community albeit geographically and legally disparate (Israel).  相似文献   
78.
Capsule Changes in return date coincided with marked changes in population size that probably resulted in fluctuating competition for nest-sites.

Aims To document the changes in return dates over a 44-year period and to identify the factors associated with these changes.

Methods We compared changes in return date at Shetland colonies with those for the Isle of May, southeast Scotland, and with the available information on population size, the abundance of some fish species eaten by Common Guillemots and large-scale changes in the oceanography and climate of the eastern Atlantic as reflected by the winter index of the North Atlantic Oscillation (NAO).

Results Common Guillemots normally return to colonies in Shetland in late winter. However, during the 1960s return dates became gradually earlier with birds present from early October. Autumn return remained the norm for about ten years after which return dates gradually reverted back to late winter. In contrast, Common Guillemots on the Isle of May, 400 km south of Shetland, showed no marked shift, returning in October each year. There was a strong negative correlation between date of return of Shetland birds and population size, whereas on the Isle of May birds came back earlier when there was a large positive winter NAO index. There was no convincing evidence that changes in wintering areas or fish abundance influenced when birds returned to the colonies, although the fish data may not have been collected on the correct spatial scale.

Conclusion Competition for high quality nest-sites is the most likely reason for Common Guillemots returning to the colonies during the autumn and winter.  相似文献   
79.
Joseph Webster 《Ethnos》2013,78(3):380-402
In Gamrie (a Scottish fishing village of 700 people and 6 Protestant churches), local experiences of ‘divine providence’ and ‘demonic attack’ abound. Bodily fluids, scraps of paper, video cassettes and prawn trawlers were immanent carriers of divine and demonic activity. Viewed through the lens of Weberian social theory, the experiences of Scottish fisher families show how the life of the Christian resembles an enchanted struggle between God and the Devil with the Christian placed awkwardly in-between. Because, locally, ‘there is no such thing as coincidence’, these Christians expected to experience both the transcendent ordering of life by divine providence through God's immanence and the transcendent disordering of life by demonic attack through the Devil's immanence. Where this ordering and disordering frequently occurred through everyday objects, seemingly mundane events – being given a washing machine or feeling sleepy in church – were experienced as material indexes of spiritual reality. Drawing on the work of Cannell (on transcendence), Keane (on indexicality) and Wagner (on symbolic obviation), this paper argues that attending to the materiality of Scottish Protestantism better equips the anthropology of religion to understand Christian experience by positing immanence as a kind of transcendence and transcendence as a kind of immanence.  相似文献   
80.
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