全文获取类型
收费全文 | 756篇 |
免费 | 3篇 |
国内免费 | 3篇 |
专业分类
系统科学 | 18篇 |
教育与普及 | 1篇 |
理论与方法论 | 5篇 |
现状及发展 | 95篇 |
研究方法 | 105篇 |
综合类 | 486篇 |
自然研究 | 52篇 |
出版年
2018年 | 5篇 |
2017年 | 6篇 |
2016年 | 3篇 |
2015年 | 4篇 |
2014年 | 2篇 |
2013年 | 3篇 |
2012年 | 53篇 |
2011年 | 117篇 |
2010年 | 11篇 |
2009年 | 6篇 |
2008年 | 48篇 |
2007年 | 60篇 |
2006年 | 58篇 |
2005年 | 58篇 |
2004年 | 48篇 |
2003年 | 45篇 |
2002年 | 51篇 |
2001年 | 14篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1991年 | 3篇 |
1990年 | 3篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 2篇 |
1986年 | 10篇 |
1985年 | 10篇 |
1984年 | 13篇 |
1983年 | 9篇 |
1982年 | 7篇 |
1981年 | 4篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1973年 | 5篇 |
1972年 | 2篇 |
1971年 | 8篇 |
1970年 | 9篇 |
1968年 | 4篇 |
1967年 | 5篇 |
1966年 | 6篇 |
1965年 | 5篇 |
1961年 | 2篇 |
1958年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有762条查询结果,搜索用时 15 毫秒
1.
Beardsley T 《Nature》1985,313(6002):421
A physiologist and the chief veterinarian at the University of Western Ontario are facing possible prosecution for cruelty to animals because of an experiment in which a baboon was kept for several months in a restraining chair. The case is believed to be the first in Canada brought against researchers following complaints by an animal welfare group. The experiment was supported by the Canadian Medical Research Council and the Ontario Heart and Stroke Fund, and the protocol and laboratory were approved by the Canadian Council on Animal Care. 相似文献
2.
Beardsley T 《Nature》1985,313(5998):85
The dispute among federal agencies over regulation of commercial biotechnology has been addressed in the Federal Register by the Office of Science and Technology Policy (OSTP), which says the Recombinant DNA Advisory Committee of the National Institutes of Health should continue to oversee biomedical research in this area, while the Environmental Protection Agency should formulate regulatory policies for industry and resolve such issues as when a microorganism constitutes a new chemical substance. OSTP also proposes four new independent expert committees to advise the EPA, the Food and Drug Administration, the Department of Agriculture, and the National Science Foundation, and a Biotechnology Science Board to coordinate the work of the committees. 相似文献
3.
This paper is a case study of a major System Dynamics (SD) intervention in policy design in the Australian Taxation Office. The project, which lasted two years, tested a number of hypotheses regarding the application of Stafford Beer's Viable Systems Model (VSM) to the structure and process of the project. The paper reports that the success of the VSM within the ATO was qualified by political changes in the external environment. 相似文献
4.
5.
Anne Berna François Bernier Eric Chabrière Mikael Elias Ken Scott Andrew Suh 《Cellular and molecular life sciences : CMLS》2009,66(14):2205-2218
DING proteins, identified mainly by their eponymous N-terminal sequences, are ubiquitous in living organisms. Amongst bacteria,
they are common in pseudomonads, and have been characterised with respect to genetics and structure. They form part of a wider
family of phosphate-binding proteins, with emerging roles in phosphate acquisition and pathogenicity. Many DING proteins have
been isolated in eukaryotes, in which they have been associated with very diverse biological activities, often in the context
of possible signalling roles. Disease states in which DING proteins have been implicated include rheumatoid arthritis, lithiasis,
atherosclerosis, some tumours and tumour-associated cachexia, and bacterial and viral adherence. Complete genetic and structural
characterisation of eukaryotic DING genes and proteins is still lacking, though the phosphate-binding site seems to be conserved.
Whether as bacterial proteins related to bacterial pathogenicity, or as eukaryotic components of biochemical signalling systems,
DING proteins require further study.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
6.
Materials showing simultaneous ferroelectric and magnetic ordering are attracting a great deal of interest because of their unusual physics and potential applications. Hemberger et al. have reported relaxor-like dielectric properties and colossal magnetocapacitance (in excess of 500%) for the cubic spinel compound CdCr2S4 and related isomorphs, concluding that CdCr2S4 is a multiferroic relaxor. We argue here, however, that their results might also be explained by a conductive artefact. 相似文献
7.
Low beta diversity of herbivorous insects in tropical forests 总被引:1,自引:0,他引:1
Novotny V Miller SE Hulcr J Drew RA Basset Y Janda M Setliff GP Darrow K Stewart AJ Auga J Isua B Molem K Manumbor M Tamtiai E Mogia M Weiblen GD 《Nature》2007,448(7154):692-695
Recent advances in understanding insect communities in tropical forests have contributed little to our knowledge of large-scale patterns of insect diversity, because incomplete taxonomic knowledge of many tropical species hinders the mapping of their distribution records. This impedes an understanding of global biodiversity patterns and explains why tropical insects are under-represented in conservation biology. Our study of approximately 500 species from three herbivorous guilds feeding on foliage (caterpillars, Lepidoptera), wood (ambrosia beetles, Coleoptera) and fruit (fruitflies, Diptera) found a low rate of change in species composition (beta diversity) across 75,000 square kilometres of contiguous lowland rainforest in Papua New Guinea, as most species were widely distributed. For caterpillars feeding on large plant genera, most species fed on multiple host species, so that even locally restricted plant species did not support endemic herbivores. Large plant genera represented a continuously distributed resource easily colonized by moths and butterflies over hundreds of kilometres. Low beta diversity was also documented in groups with differing host specificity (fruitflies and ambrosia beetles), suggesting that dispersal limitation does not have a substantial role in shaping the distribution of insect species in New Guinea lowland rainforests. Similar patterns of low beta diversity can be expected in other tropical lowland rainforests, as they are typically situated in the extensive low basins of major tropical rivers similar to the Sepik-Ramu region of New Guinea studied here. 相似文献
8.
9.
Koebel CM Vermi W Swann JB Zerafa N Rodig SJ Old LJ Smyth MJ Schreiber RD 《Nature》2007,450(7171):903-907
The capacity of immunity to control and shape cancer, that is, cancer immunoediting, is the result of three processes that function either independently or in sequence: elimination (cancer immunosurveillance, in which immunity functions as an extrinsic tumour suppressor in naive hosts); equilibrium (expansion of transformed cells is held in check by immunity); and escape (tumour cell variants with dampened immunogenicity or the capacity to attenuate immune responses grow into clinically apparent cancers). Extensive experimental support now exists for the elimination and escape processes because immunodeficient mice develop more carcinogen-induced and spontaneous cancers than wild-type mice, and tumour cells from immunodeficient mice are more immunogenic than those from immunocompetent mice. In contrast, the equilibrium process was inferred largely from clinical observations, including reports of transplantation of undetected (occult) cancer from organ donor into immunosuppressed recipients. Herein we use a mouse model of primary chemical carcinogenesis and demonstrate that equilibrium occurs, is mechanistically distinguishable from elimination and escape, and that neoplastic cells in equilibrium are transformed but proliferate poorly in vivo. We also show that tumour cells in equilibrium are unedited but become edited when they spontaneously escape immune control and grow into clinically apparent tumours. These results reveal that, in addition to destroying tumour cells and sculpting tumour immunogenicity, the immune system of a naive mouse can also restrain cancer growth for extended time periods. 相似文献
10.
Sexually antagonistic genetic variation for fitness in red deer 总被引:1,自引:0,他引:1
Foerster K Coulson T Sheldon BC Pemberton JM Clutton-Brock TH Kruuk LE 《Nature》2007,447(7148):1107-1110
Evolutionary theory predicts the depletion of genetic variation in natural populations as a result of the effects of selection, but genetic variation is nevertheless abundant for many traits that are under directional or stabilizing selection. Evolutionary geneticists commonly try to explain this paradox with mechanisms that lead to a balance between mutation and selection. However, theoretical predictions of equilibrium genetic variance under mutation-selection balance are usually lower than the observed values, and the reason for this is unknown. The potential role of sexually antagonistic selection in maintaining genetic variation has received little attention in this debate, surprisingly given its potential ubiquity in dioecious organisms. At fitness-related loci, a given genotype may be selected in opposite directions in the two sexes. Such sexually antagonistic selection will reduce the otherwise-expected positive genetic correlation between male and female fitness. Both theory and experimental data suggest that males and females of the same species may have divergent genetic optima, but supporting data from wild populations are still scarce. Here we present evidence for sexually antagonistic fitness variation in a natural population, using data from a long-term study of red deer (Cervus elaphus). We show that male red deer with relatively high fitness fathered, on average, daughters with relatively low fitness. This was due to a negative genetic correlation between estimates of fitness in males and females. In particular, we show that selection favours males that carry low breeding values for female fitness. Our results demonstrate that sexually antagonistic selection can lead to a trade-off between the optimal genotypes for males and females; this mechanism will have profound effects on the operation of selection and the maintenance of genetic variation in natural populations. 相似文献