全文获取类型
收费全文 | 389篇 |
免费 | 0篇 |
专业分类
系统科学 | 1篇 |
教育与普及 | 1篇 |
理论与方法论 | 2篇 |
现状及发展 | 60篇 |
研究方法 | 33篇 |
综合类 | 254篇 |
自然研究 | 38篇 |
出版年
2019年 | 1篇 |
2018年 | 1篇 |
2017年 | 1篇 |
2015年 | 1篇 |
2014年 | 1篇 |
2013年 | 3篇 |
2012年 | 8篇 |
2011年 | 57篇 |
2010年 | 2篇 |
2009年 | 1篇 |
2008年 | 6篇 |
2007年 | 10篇 |
2006年 | 9篇 |
2005年 | 12篇 |
2004年 | 12篇 |
2003年 | 14篇 |
2002年 | 16篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 11篇 |
1995年 | 1篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 15篇 |
1989年 | 5篇 |
1988年 | 7篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 13篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1981年 | 4篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 4篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1971年 | 9篇 |
1970年 | 15篇 |
1969年 | 8篇 |
1968年 | 7篇 |
1967年 | 8篇 |
1966年 | 5篇 |
1965年 | 11篇 |
1963年 | 1篇 |
排序方式: 共有389条查询结果,搜索用时 898 毫秒
151.
152.
Anderson AJ 《Nature》1971,229(5286):547-548
153.
154.
Geophysical hotspots have been attributed to partially molten asthenosphere, fertile blobs, small-scale convection and upwellings driven by core heat. Most are short-lived or too close together to be deeply seated, and do not have anomalous heat flow or temperature; many are related to tectonic features. Bourdon et al. investigate the dynamics of mantle plumes from uranium-series geochemistry and interpret their results as evidence for thermal plumes. Here we show why alternative mechanisms of upwelling and melting should be considered. 相似文献
155.
The lakes of Titan 总被引:1,自引:0,他引:1
Stofan ER Elachi C Lunine JI Lorenz RD Stiles B Mitchell KL Ostro S Soderblom L Wood C Zebker H Wall S Janssen M Kirk R Lopes R Paganelli F Radebaugh J Wye L Anderson Y Allison M Boehmer R Callahan P Encrenaz P Flamini E Francescetti G Gim Y Hamilton G Hensley S Johnson WT Kelleher K Muhleman D Paillou P Picardi G Posa F Roth L Seu R Shaffer S Vetrella S West R 《Nature》2007,445(7123):61-64
The surface of Saturn's haze-shrouded moon Titan has long been proposed to have oceans or lakes, on the basis of the stability of liquid methane at the surface. Initial visible and radar imaging failed to find any evidence of an ocean, although abundant evidence was found that flowing liquids have existed on the surface. Here we provide definitive evidence for the presence of lakes on the surface of Titan, obtained during the Cassini Radar flyby of Titan on 22 July 2006 (T16). The radar imaging polewards of 70 degrees north shows more than 75 circular to irregular radar-dark patches, in a region where liquid methane and ethane are expected to be abundant and stable on the surface. The radar-dark patches are interpreted as lakes on the basis of their very low radar reflectivity and morphological similarities to lakes, including associated channels and location in topographic depressions. Some of the lakes do not completely fill the depressions in which they lie, and apparently dry depressions are present. We interpret this to indicate that lakes are present in a number of states, including partly dry and liquid-filled. These northern-hemisphere lakes constitute the strongest evidence yet that a condensable-liquid hydrological cycle is active in Titan's surface and atmosphere, in which the lakes are filled through rainfall and/or intersection with the subsurface 'liquid methane' table. 相似文献
156.
Establishing the conditions that promote the evolution of reproductive isolation and speciation has long been a goal in evolutionary biology. In ecological speciation, reproductive isolation between populations evolves as a by-product of divergent selection and the resulting environment-specific adaptations. The leading genetic model of reproductive isolation predicts that hybrid inferiority is caused by antagonistic epistasis between incompatible alleles at interacting loci. The fundamental link between divergent adaptation and reproductive isolation through genetic incompatibilities has been predicted, but has not been directly demonstrated experimentally. Here we empirically tested key predictions of speciation theory by evolving the initial stages of speciation in experimental populations of the yeast Saccharomyces cerevisiae. After replicate populations adapted to two divergent environments, we consistently observed the evolution of two forms of postzygotic isolation in hybrids: reduced rate of mitotic reproduction and reduced efficiency of meiotic reproduction. This divergent selection resulted in greater reproductive isolation than parallel selection, as predicted by the ecological speciation theory. Our experimental system allowed controlled comparison of the relative importance of ecological and genetic isolation, and we demonstrated that hybrid inferiority can be ecological and/or genetic in basis. Overall, our results show that adaptation to divergent environments promotes the evolution of reproductive isolation through antagonistic epistasis, providing evidence of a plausible common avenue to speciation and adaptive radiation in nature. 相似文献
157.
A polymorphic DNA marker genetically linked to Huntington's disease 总被引:32,自引:0,他引:32
J F Gusella N S Wexler P M Conneally S L Naylor M A Anderson R E Tanzi P C Watkins K Ottina M R Wallace A Y Sakaguchi 《Nature》1983,306(5940):234-238
Family studies show that the Huntington's disease gene is linked to a polymorphic DNA marker that maps to human chromosome 4. The chromosomal localization of the Huntington's disease gene is the first step in using recombinant DNA technology to identify the primary genetic defect in this disorder. 相似文献
158.
159.
160.
Haubensak W Kunwar PS Cai H Ciocchi S Wall NR Ponnusamy R Biag J Dong HW Deisseroth K Callaway EM Fanselow MS Lüthi A Anderson DJ 《Nature》2010,468(7321):270-276
The role of different amygdala nuclei (neuroanatomical subdivisions) in processing Pavlovian conditioned fear has been studied extensively, but the function of the heterogeneous neuronal subtypes within these nuclei remains poorly understood. Here we use molecular genetic approaches to map the functional connectivity of a subpopulation of GABA-containing neurons, located in the lateral subdivision of the central amygdala (CEl), which express protein kinase C-δ (PKC-δ). Channelrhodopsin-2-assisted circuit mapping in amygdala slices and cell-specific viral tracing indicate that PKC-δ(+) neurons inhibit output neurons in the medial central amygdala (CEm), and also make reciprocal inhibitory synapses with PKC-δ(-) neurons in CEl. Electrical silencing of PKC-δ(+) neurons in vivo suggests that they correspond to physiologically identified units that are inhibited by the conditioned stimulus, called CEl(off) units. This correspondence, together with behavioural data, defines an inhibitory microcircuit in CEl that gates CEm output to control the level of conditioned freezing. 相似文献