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   11888篇
  免费   45篇
  国内免费   71篇
系统科学   68篇
丛书文集   29篇
教育与普及   21篇
理论与方法论   35篇
现状及发展   5550篇
研究方法   608篇
综合类   5565篇
自然研究   128篇
  2012年   217篇
  2011年   318篇
  2010年   80篇
  2009年   75篇
  2008年   217篇
  2007年   257篇
  2006年   265篇
  2005年   247篇
  2004年   216篇
  2003年   210篇
  2002年   253篇
  2001年   442篇
  2000年   432篇
  1999年   291篇
  1992年   239篇
  1991年   195篇
  1990年   217篇
  1989年   193篇
  1988年   204篇
  1987年   220篇
  1986年   190篇
  1985年   254篇
  1984年   227篇
  1983年   173篇
  1982年   178篇
  1981年   180篇
  1980年   172篇
  1979年   408篇
  1978年   343篇
  1977年   257篇
  1976年   290篇
  1975年   261篇
  1974年   277篇
  1973年   225篇
  1972年   247篇
  1971年   311篇
  1970年   381篇
  1969年   263篇
  1968年   310篇
  1967年   293篇
  1966年   253篇
  1965年   178篇
  1964年   99篇
  1959年   88篇
  1958年   163篇
  1957年   103篇
  1956年   91篇
  1955年   86篇
  1954年   79篇
  1948年   64篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
J. D. Trout has recently developed a new defense of scientific realism, a new version of the No Miracles Argument. I critically evaluate Trout's novel defense of realism. I argue that Trout's argument for scientific realism and the related explanation for the success of science are self-defeating. In the process of arguing against the traditional realist strategies for explaining the success of science, he inadvertently undermines his own argument.  相似文献   
2.
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobiaknysnaensis (Krauss), ‘Assimineacapensis (Sowerby) and ‘Assimineaglobulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere.  相似文献   
3.
This paper undertakes a comprehensive examination of 10 measures of core inflation and evaluates which measure produces the best forecast of headline inflation out‐of‐sample. We use the Personal Consumption Expenditure Price Index as our measure of inflation. We use two sets of components (17 and 50) of the Personal Consumption Expenditure Price Index to construct these core inflation measures and evaluate these measures at the three time horizons (6, 12 and 24 months) most relevant for monetary policy decisions. The best measure of core inflation for both sets of components and over all time horizons uses weights based on the first principal component of the disaggregated (component‐level) prices. Interestingly, the results vary by the number of components used; when more components are used the weights based on the persistence of each component is statistically equivalent to the weights generated by the first principal component. However, those forecasts using the persistence of 50 components are statistically worse than those generated using the first principal component of 17 components. The statistical superiority of the principal component method is due to the fact that it extracts (in the first principal component) the common source of variation in the component level prices that accurately describes trend inflation over the next 6–24 months.  相似文献   
4.
5.
6.
Most living organisms show circadian rhythms in physiology and behavior. These oscillations are generated by endogenous circadian clocks, present in virtually all cells where they control key biological processes. To study peripheral clocks in vivo, we developed an original model, the Rev-Luc mouse to follow noninvasively and longitudinally Rev-Luc oscillations in peripheral clocks using in vivo bioluminescence imaging. We found in vitro and in vivo a robust diurnal rhythm of Rev-Luc, mainly in liver, intestine, kidney and adipose tissues. We further confirmed in vivo that Rev-Luc peripheral tissues are food-entrainable oscillators, not affected by age or sex. These data strongly support the relevance of the Rev-Luc model for circadian studies, especially to investigate in vivo the establishment and the entrainment of the rhythm throughout ontogenesis. We then showed that Rev-Luc expression develops dynamically and gradually, both in amplitude and in phase, during fetal and postnatal development. We also demonstrate for the first time that the immature peripheral circadian system of offspring in utero is mainly entrained by maternal cues from feeding regimen. The prenatal entrainment will also differentially determine the Rev-Luc expression in pups before weaning underlining the importance of the maternal chrononutrition on the circadian system entrainment of the offspring.  相似文献   
7.
8.
9.
Tissue-specific and reversible RNA interference in transgenic mice   总被引:11,自引:0,他引:11  
Genetically engineered mice provide powerful tools for understanding mammalian gene function. These models traditionally rely on gene overexpression from transgenes or targeted, irreversible gene mutation. By adapting the tetracycline (tet)-responsive system previously used for gene overexpression, we have developed a simple transgenic system to reversibly control endogenous gene expression using RNA interference (RNAi) in mice. Transgenic mice harboring a tet-responsive RNA polymerase II promoter driving a microRNA-based short hairpin RNA targeting the tumor suppressor Trp53 reversibly express short hairpin RNA when crossed with existing mouse strains expressing general or tissue-specific 'tet-on' or 'tet-off' transactivators. Reversible Trp53 knockdown can be achieved in several tissues, and restoring Trp53 expression in lymphomas whose development is promoted by Trp53 knockdown leads to tumor regression. By leaving the target gene unaltered, this approach permits tissue-specific, reversible regulation of endogenous gene expression in vivo, with potential broad application in basic biology and drug target validation.  相似文献   
10.
Glycosylation constitutes one of the most important posttranslational modifications employed by biological systems to modulate protein biophysical properties. Due to the direct biochemical and biomedical implications of achieving control over protein stability and function by chemical means, there has been great interest in recent years towards the development of chemical strategies for protein glycosylation. Since current knowledge about glycoprotein biophysics has been mainly derived from the study of naturally glycosylated proteins, chemical glycosylation provides novel insights into its mechanistic understanding by affording control over glycosylation parameters. This review presents a survey of the effects that natural and chemical glycosylation have on the fundamental biophysical properties of proteins (structure, dynamics, stability, and function). This is complemented by a mechanistic discussion of how glycans achieve such effects and discussion of the implications of employing chemical glycosylation as a tool to exert control over protein biophysical properties within biochemical and biomedical applications. Received 15 December 2006; received after revision 28 March 2007; accepted 25 April 2007  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