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   349篇
  免费   3篇
系统科学   6篇
教育与普及   2篇
理论与方法论   18篇
现状及发展   115篇
研究方法   37篇
综合类   157篇
自然研究   17篇
  2017年   5篇
  2016年   7篇
  2015年   4篇
  2014年   2篇
  2013年   6篇
  2012年   20篇
  2011年   36篇
  2010年   13篇
  2008年   28篇
  2007年   18篇
  2006年   13篇
  2005年   19篇
  2004年   11篇
  2003年   14篇
  2002年   11篇
  2001年   3篇
  2000年   4篇
  1999年   4篇
  1998年   3篇
  1996年   2篇
  1992年   8篇
  1990年   2篇
  1989年   3篇
  1988年   4篇
  1987年   6篇
  1986年   2篇
  1985年   4篇
  1984年   2篇
  1983年   2篇
  1982年   3篇
  1980年   3篇
  1979年   10篇
  1978年   4篇
  1976年   3篇
  1975年   6篇
  1974年   6篇
  1973年   8篇
  1972年   3篇
  1971年   4篇
  1970年   10篇
  1969年   2篇
  1968年   7篇
  1967年   5篇
  1966年   4篇
  1965年   2篇
  1963年   2篇
  1960年   1篇
  1957年   3篇
  1948年   1篇
  1947年   1篇
排序方式: 共有352条查询结果,搜索用时 62 毫秒
91.
Nineteenth-century spiritism was a blend of religious elements, the philosophy of mind, science and popular science and contacts with extraterrestrials were a commonplace phenomenon during spiritistic séances. Using the example of Carl du Prel (1839–1899) I show how his comprehensive mystic philosophy originated in a theory of extraterrestrial life. Carl du Prel used a Darwinian and monistic framework, theories of the unconscious and a Neo-Kantian epistemology to formulate a philosophy of astronomy and extraterrestrial life. He claimed that the mechanism of Darwinian selection is responsible for the distribution of stars and the orbits of the planets. In his speculations on the nature of extraterrestrial life he used the concept of organ projection to argue that technical solutions on earth will be realized organically on other planets and claimed that superior extraterrestrials have quantitatively and qualitatively different senses and thus different forms of intuition. A comparison with Camille Flammarion, spiritist and populariser of astronomy, demonstrates the contextual complexities of spiritism. In contrast to du Prel’s sober Neo-Kantian philosophical speculations, Flammarion was a late proponent of a French esoteric tradition that was rooted in romantic socialism, painted grand cosmological vistas and emphasized reincarnation. I put forward the hypothesis that current discourses on extraterrestrial life are affected by the spiritist tradition mainly through the ‘Golden Age’ science fiction literature of the 1940s and 50s and its successors. However, neither Carl du Prel nor Camille Flammarion contributed significantly to this tradition, which is mainly shaped by the psychical research of J. B. Rhine.  相似文献   
92.
Theoretical studies have shown that variation in density regulation strongly influences population dynamics, yet our understanding of factors influencing the strength of density dependence in natural populations still is limited. Consequently, few general hypotheses have been advanced to explain the large differences between species in the magnitude of population fluctuations. One reason for this is that the detection of density regulation in population time series is complicated by time lags induced by the life history of species that make it difficult to separate the relative contributions of intrinsic and extrinsic factors to the population dynamics. Here we use population time series for 23 bird species to estimate parameters of a stochastic density-dependent age-structured model. We show that both the strength of total density dependence in the life history and the magnitude of environmental stochasticity, including transient fluctuations in age structure, increase with generation time. These results indicate that the relationships between demographic and life-history traits in birds translate into distinct population dynamical patterns that are apparent only on a scale of generations.  相似文献   
93.
Cheung VG  Spielman RS  Ewens KG  Weber TM  Morley M  Burdick JT 《Nature》2005,437(7063):1365-1369
To study the genetic basis of natural variation in gene expression, we previously carried out genome-wide linkage analysis and mapped the determinants of approximately 1,000 expression phenotypes. In the present study, we carried out association analysis with dense sets of single-nucleotide polymorphism (SNP) markers from the International HapMap Project. For 374 phenotypes, the association study was performed with markers only from regions with strong linkage evidence; these regions all mapped close to the expressed gene. For a subset of 27 phenotypes, analysis of genome-wide association was performed with >770,000 markers. The association analysis with markers under the linkage peaks confirmed the linkage results and narrowed the candidate regulatory regions for many phenotypes with strong linkage evidence. The genome-wide association analysis yielded highly significant results that point to the same locations as the genome scans for about 50% of the phenotypes. For one candidate determinant, we carried out functional analyses and confirmed the variation in cis-acting regulatory activity. Our findings suggest that association studies with dense SNP maps will identify susceptibility loci or other determinants for some complex traits or diseases.  相似文献   
94.
Mapping the Hawaiian plume conduit with converted seismic waves   总被引:4,自引:0,他引:4  
Li X  Kind R  Priestley K  Sobolev SV  Tilmann F  Yuan X  Weber M 《Nature》2000,405(6789):938-941
The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s(-1)) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of approximately 300 degrees C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100 degrees C higher temperature for the Hawaiian plume.  相似文献   
95.
Kinetics of BRCA1 regulation in response to UVC radiation   总被引:1,自引:0,他引:1  
To investigate changes in BRCA1 following DNA damage, we exposed MCF-7 cells to increasing doses of ultraviolet C. We observed an increase in BRCA1 protein levels above 78 J/m2. This increase was observed as early as 5 min after irradiation. BRCA1 levels were then observed to decrease after 2 h, consistent with the previously published data. By pretreating with cycloheximide prior to irradiation, we observed a decrease in the protein half-life, from 3.5 h to 53 min, suggesting that a decrease in protein half-life may cause the lower levels of BRCA1 after irradiation. We also observed an increase in BRCA1 mRNA within 15 min of irradiation, followed by a decrease after 4 h. These data suggest that newly translated protein may contribute to increases in BRCA1 protein levels. The very rapid changes in BRCA1 support its role as a sensor of DNA damage, as opposed to being a repair gene. Received 6 April 2000; received after revision 23 May 2000; accepted 23 May 2000  相似文献   
96.
In vivo degradation of mutant lac repressor   总被引:19,自引:0,他引:19  
T Platt  J H Miller  K Weber 《Nature》1970,228(5277):1154-1156
  相似文献   
97.
Summary Fractionated X-irradiation of mouse fetuses on gestation days 11–13 resulted in a significantly increased postnatal mortality of female litters. This occurred only at 3×110 rad, which was the threshold for the formation of typical neuroepithelial, rosette-like malformations.We are greatly indebted for the technical assistance of Miss C. Gutmann, Mrs I. Hempfling and Mrs K. Pfaff.  相似文献   
98.
99.
Zusammenfassung Acetylcholin wirkt als Aktivator auf Oxydationsreaktionen mit Wasserstoffperoxyd in alkalischer Lösung. Solche Wirkungen auf die Chemilumineszenz des Luminols (3-Aminophthalhydrazid), auf die Oxydation des Indols, Benzidins und o-Dianizidins wurden durch Messungen der Lumineszenzintensität (), bzw, der Extinktion (E) näher untersucht.  相似文献   
100.
Summary Mammary glands of the white mouse in different stages of lactation were fixed in Carnoy's fluid and the sections stained with Gallocyanin-chromalum für microscopic observation. After dislocation of the nucleolus to the nuclear membrane or the formation of a heterochromatic pathway between nucleolus and nuclear membrane the extrusion of nucleolar substance into the cytoplasm was observed.  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