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   118篇
  免费   1篇
  2023年   4篇
  2021年   1篇
  2020年   4篇
  2019年   5篇
  2018年   1篇
  2017年   1篇
  2016年   1篇
  2014年   4篇
  2013年   40篇
  2012年   2篇
  2011年   4篇
  2010年   2篇
  2009年   4篇
  2008年   3篇
  2007年   2篇
  2006年   1篇
  2004年   5篇
  2000年   2篇
  1999年   1篇
  1998年   3篇
  1997年   2篇
  1996年   2篇
  1993年   1篇
  1990年   1篇
  1989年   2篇
  1987年   3篇
  1986年   1篇
  1984年   1篇
  1983年   6篇
  1982年   1篇
  1981年   2篇
  1976年   3篇
  1975年   1篇
  1974年   1篇
  1971年   2篇
排序方式: 共有119条查询结果,搜索用时 31 毫秒
1.
2.
3.
We have investigated the survival of PCR-amplifiable mitochondrial and nuclear DNA in a small number of modern and medieval bovine leather samples. The results of this preliminary investigation demonstrate that, while no nuclear DNA can be PCR-amplified from any of the specimens, mitochondrial DNA can be amplified from all samples. To investigate this contrasting pattern of DNA survival further, we have vegetable-tanned cow skin in our own laboratory, and directly assayed the survival of PCR-amplifiable mitochondrial and nuclear DNA at each step of the process. The results indicate that nuclear DNA is reduced to sub-amplifiable levels as a result of the tanning baths, whereas amplifiable mitochondrial DNA survives the complete process. Our results suggest that old and archaeological bovine leather may represent a useful source of genetic information, although this information will most likely be limited to that which can be gained from mitochondrial DNA.  相似文献   
4.
Under the leadership of Matteo Salvini, the Lega Nord has shifted away from its previous political identity as a voice for Italy’s north and has placed hostility towards the policies and institutions of the European Union (EU) at the heart of its rhetoric. Nowadays, the enemy is Rome no longer: it is Brussels, European institutions, and the threat to the national sovereignty posed by the EU. Borrowing from the Italian political philosopher Nicola Matteucci, we would describe Salvini’s Lega as a ‘populist insurgency’. That is to say, it is a populist party that marries the traditional populist evocation of the virtues of the people against the corrupt elites, with a pervasive glibness of analysis.  相似文献   
5.
6.
7.
8.
9.
Systematic collecting from fluvial late Pleistocene deposits from the Darling Downs, southeast Queensland, Australia, has led to the recovery of the first fossil frogs from the region, ail from the Myobatrachidae, a family of ground dwelling and burrowing frogs. The most common species recovered, Limnodynastes tasmaniensis, is extant on the Darling Downs. The fossil taxa include species whose extant populations inhabit arid zones(Limnodynastes sp. cf. L. spenceri), montane forests (Kyarranus spp.), and open woodlands (Neobatrachus sudelli), and indicate the existence of a mosaic of habitats during the Pleistocene. The absence of the Hylidae (tree frogs), a family common throughout the Darling Downs today, may be explained by a taphonomic bias that favours non-arboreal forms. Alternatively, hylids may have been rare or absent on the Darling Downs during the Pleistocene.  相似文献   
10.
The provision of adequate primary health care in developing countries is often troublesome. The problem is to provide a sufficient number of facilities to be geographically accessible, yet few enough to be properly stocked and staffed. In many less developed countries accessibility problems are exacerbated by extensive rainy seasons in which travel is only possible on paved roads. Using the covering tour model we investigate the use of mobile facilities to resolve this dilemma in Suhum District, Ghana. The model minimizes a mobile facility's travel while serving all population centers within range of a feasible stop. Computational results show that in the rainy season the model cannot provide full coverage; over six percent of the population is beyond a covering distance of eight kilometers. In the dry season, 99 percent of the population can be served by a tour at a covering distance of seven kilometers.Beyond a distance of four kilometers, the dry season problem becomes a trade-off between the distance traveled by healthcare patrons and mobile facilities. These results illustrate the importance of flexibility of mobile systems: if accessibility cannot be provided in all seasons it may still be provided at favorable times of the year.  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