全文获取类型
收费全文 | 318455篇 |
免费 | 36818篇 |
国内免费 | 158篇 |
出版年
2018年 | 2870篇 |
2017年 | 2687篇 |
2016年 | 3825篇 |
2015年 | 5238篇 |
2014年 | 6277篇 |
2013年 | 8304篇 |
2012年 | 9804篇 |
2011年 | 10053篇 |
2010年 | 6605篇 |
2009年 | 6156篇 |
2008年 | 8878篇 |
2007年 | 8936篇 |
2006年 | 8689篇 |
2005年 | 8350篇 |
2004年 | 8249篇 |
2003年 | 7871篇 |
2002年 | 7584篇 |
2001年 | 17489篇 |
2000年 | 17619篇 |
1999年 | 13525篇 |
1998年 | 4121篇 |
1997年 | 4324篇 |
1996年 | 4038篇 |
1995年 | 3651篇 |
1994年 | 3636篇 |
1993年 | 3682篇 |
1992年 | 10528篇 |
1991年 | 10421篇 |
1990年 | 9890篇 |
1989年 | 9733篇 |
1988年 | 9089篇 |
1987年 | 8442篇 |
1986年 | 7625篇 |
1985年 | 7484篇 |
1984年 | 5927篇 |
1983年 | 5172篇 |
1982年 | 3707篇 |
1981年 | 3260篇 |
1980年 | 3106篇 |
1979年 | 5410篇 |
1978年 | 4176篇 |
1977年 | 3793篇 |
1976年 | 3348篇 |
1975年 | 3815篇 |
1974年 | 3980篇 |
1973年 | 3911篇 |
1972年 | 3436篇 |
1971年 | 3204篇 |
1970年 | 2839篇 |
1969年 | 2760篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
R K Butlin T Tregenza 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》1998,353(1366):187-198
Species are the units used to measure ecological diversity and alleles are the units of genetic diversity. Genetic variation within and among species has been documented most extensively using allozyme electrophoresis. This reveals wide differences in genetic variability within, and genetic distances among, species, demonstrating that species are not equivalent units of diversity. The extent to which the pattern observed for allozymes can be used to infer patterns of genetic variation in quantitative traits depends on the forces generating and maintaining variability. Allozyme variation is probably not strictly neutral but, nevertheless, heterozygosity is expected to be influenced by population size and genetic distance will be affected by time since divergence. The same is true for quantitative traits influenced by many genes and under weak stabilizing selection. However, the limited data available suggest that allozyme variability is a poor predictor of genetic variation in quantitative traits within populations. It is a better predictor of general phenotypic divergence and of postzygotic isolation between populations or species, but is only weakly correlated with prezygotic isolation. Studies of grasshopper and planthopper mating signal variation and assortative mating illustrate how these characters evolve independently of general genetic and morphological variation. The role of such traits in prezygotic isolation, and hence speciation, means that they will contribute significantly to the diversity of levels of genetic variation within and among species. 相似文献
52.
Bidens cordylocarpa is a high polyploid species restricted in distribution to stream sides in the mountains of Jalisco, Mexico. The morphologically
enigmatic species was originally described as a member of the genus Coreopsis, but later transferred to Bidens, largely because the involucral bracts appear most similar to Bidens. Characters of the cypselae, often useful in generic placement, are of no value for this species because the fruits have
features not detected in either Bidens or Coreopsis. Sequences from the internal transcribed spacer region of nuclear ribosomal DNA (ITS) were used to assess the relationships
of Bidens cordylocarpa. The molecular phylogeny places B. cordylocarpa in a strongly supported clade of Mexican and South American Bidens, and provides more definitive evidence of relationships than morphology, chromosome number, or secondary chemistry. Molecular,
morphological, and chromosomal data suggest that B. cordylocarpa is an ancient polyploid, perhaps the remnant of a polyploid complex.
Received August 28, 2000 Accepted February 11, 2001 相似文献
53.
Ramakrishna A. Bangaru Yatish K. Bansal A. R. M. Rao T. P. Gandhi 《Journal of chromatography. B, Analytical technologies in the biomedical and life sciences》2000,739(2)
A rapid, simple and sensitive reversed-phase high-performance liquid chromatographic (HPLC) method has been developed for the measurement of acyclovir concentrations in human plasma and its use in bioavailability studies is evaluated. Unchanged acyclovir has been quantified without the introduction of an internal standard using the present method. Human plasma proteins were selectively precipitated by the addition of 7% perchloric acid to spiked plasma samples or to the plasma samples obtained after acyclovir administration to human volunteers and the mixture was spun at 1000 g for 10 min. The supernatant was directly injected into a Novaflex C18 column and detected at 254 nm. The mobile phase consisted of octane sulfonic acid buffer (pH 2.5) and methanol (92:08). The limit of quantitation for acyclovir in plasma was 20 ng/ml, which enabled the determination of the area under the curve (AUC) more precisely, that is, it is much closer to its extrapolated value. The present method has been successfully applied to samples from bioavailability studies. 相似文献
54.
55.
N N Alipov G I Kositski? T E Kuznetsova 《Biulleten' eksperimental'no? biologii i meditsiny》1985,99(2):169-171
Experiments on frogs were performed to examine the effect of the M-cholinomimetic pilocarpine on the heart. It was discovered that at concentrations of 10(-15)--10(-5) g/ml pilocarpine exerted only an adverse chronotropic effect on the perfused heart. When applied at a concentration of 10(-4) g/ml the drug produced a negative as well as a positive chronotropic effect. The latter occurred spasmodically (without progressive rise in the heart rate) in association with a slow heart rate. In some experiments such effects were preceded by a certain deceleration of the heart. In experiments with positive chronotropic effects, arrhythmias and sinoatrial dissociation were observed sometimes. Experiments with recording of the electrograms of the sinuses and lower parts showed that such effects were caused not by pacemaker acceleration but by the removal of the blockade of conduction, between the pacemaker and the atria. As far as the pacemaker is concerned, pilocarpine exerted only a negative chronotropic effect. 相似文献
56.
Wei CHEN Liqing PENG Lichun JIANG David A.PIKE Christopher R.FRIESEN Gregory BROWN 《亚洲两栖爬行动物研究(英文版)》2018,(1)
Environmental unpredictability can influence strategies of maternal investment among eggs within a clutch. Models predict that breeding females should adopt a diversified bet-hedging strategy in unpredictable environments, but empirical field evidence from Asia is scarce. Here we tested this hypothesis by exploring spatial patterns in egg size along an altitudinal gradient in a frog species(Rana kukunoris) inhabiting the Tibetan Plateau. Within-clutch variability in egg size increased as the environment became variable(e.g., lower mean monthly temperature and mean monthly rainfall at higher altitudes), and populations in environments with more unpredictable rainfall produced eggs that were smaller and more variable in size. We provide support for a diversified bet-hedging strategy in high-altitude environments, which experience dynamic weather patterns and therefore are of unpredictable environmental quality. This strategy may be an adaptive response to lower environmental quality and higher unpredictable environmental variance. Such a strategy should increase the likelihood of breeding success and maximize maternal lifetime fitness by producing offspring that are adapted to current environmental conditions. We speculate that in high-altitude environments prone to physical disturbance, breeding females are unable to consistently produce the optimal egg size due to physiological constraints imposed by environmental conditions(e.g., duration of the active season, food availability). Species and populations whose breeding strategies are adapted to cope with uncertain environmental conditions by adjusting offspring size and therefore quality show a remarkable degree of ability to cope with future climatic changes. 相似文献
57.
58.
59.
60.