全文获取类型
收费全文 | 2906篇 |
免费 | 301篇 |
国内免费 | 342篇 |
出版年
2023年 | 95篇 |
2022年 | 51篇 |
2021年 | 73篇 |
2020年 | 138篇 |
2019年 | 123篇 |
2018年 | 111篇 |
2017年 | 126篇 |
2016年 | 130篇 |
2015年 | 115篇 |
2014年 | 133篇 |
2013年 | 194篇 |
2012年 | 137篇 |
2011年 | 120篇 |
2010年 | 116篇 |
2009年 | 147篇 |
2008年 | 163篇 |
2007年 | 169篇 |
2006年 | 122篇 |
2005年 | 99篇 |
2004年 | 112篇 |
2003年 | 101篇 |
2002年 | 79篇 |
2001年 | 91篇 |
2000年 | 89篇 |
1999年 | 68篇 |
1998年 | 43篇 |
1997年 | 45篇 |
1996年 | 48篇 |
1995年 | 33篇 |
1994年 | 43篇 |
1993年 | 34篇 |
1992年 | 35篇 |
1991年 | 38篇 |
1990年 | 29篇 |
1989年 | 29篇 |
1988年 | 25篇 |
1987年 | 28篇 |
1986年 | 20篇 |
1985年 | 24篇 |
1984年 | 27篇 |
1983年 | 18篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 24篇 |
1979年 | 15篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1973年 | 9篇 |
1972年 | 5篇 |
1971年 | 9篇 |
排序方式: 共有3549条查询结果,搜索用时 15 毫秒
101.
102.
103.
104.
105.
Summary It has become increasingly common in epidemiological studies to pool specimens across subjects to achieve accurate quantitation of biomarkers and certain environmental chemicals. In this article, we consider the problem of fitting a binary regression model when an important exposure is subject to pooling. We take a regression calibration approach and derive several methods, including plug‐in methods that use a pooled measurement and other covariate information to predict the exposure level of an individual subject, and normality‐based methods that make further adjustments by assuming normality of calibration errors. Within each class we propose two ways to perform the calibration (covariate augmentation and imputation). These methods are shown in simulation experiments to effectively reduce the bias associated with the naive method that simply substitutes a pooled measurement for all individual measurements in the pool. In particular, the normality‐based imputation method performs reasonably well in a variety of settings, even under skewed distributions of calibration errors. The methods are illustrated using data from the Collaborative Perinatal Project. 相似文献
106.
CHIA‐HUNG HSIEH SIN‐CHE LEE HURNG‐YI WANG 《Zoological Journal of the Linnean Society》2011,161(3):613-632
The phylogeographical patterns of the endemic minnow, Candidia barbatus, were examined using mitochondrial (mt) cytochrome b sequences, allozyme electrophoresis, and morphology. The 23 mtDNA haplotypes recovered were grouped into six lineages (A–F). According to a molecular clock of 1.7% per million years, we estimated that the colonization of C. barbatus to Taiwan took place three million years ago with several subsequent vicariance events. In the allozyme analysis, the Bayesian approach partitioned different populations into three clusters, with clusters I, II, and III mainly found in northern, central, and southern Taiwan, respectively. Morphologically, fish from north and south of the Tropic of Cancer had distinct numbers of lateral line scales and were separated into two morphs. The cohabitation of different mtDNA lineages with high allozymic and morphological similarities suggests that interbreeding amongst them is possible. Therefore, in spite of highly differentiated mtDNA, samples from different collecting sites should be considered a single species. Based on the current population distribution and levels of divergence, we propose that both orogenic activities and marine regression during glacial epoch were the two main forces shaping population differentiation of this species. © 2011 The Linnean Society of London, Zoological Journal of the Linnean Society, 2011, 161 , 613–632. 相似文献
107.
Aim Using dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae) in a tropical land‐bridge island system, we test for the small island effect (SIE) in the species–area relationship and evaluate its effects on species richness and community composition. We also examine the determinants of species richness across island size and investigate the traits of dung beetle species in relation to their local extinction vulnerability following forest fragmentation. Location Lake Kenyir, a hydroelectric reservoir in north‐eastern Peninsular Malaysia. Methods We sampled dung beetles using human dung baited pitfall traps on 24 land‐bridge islands and three mainland sites. We used regression tree analyses to test for the SIE, as well as species traits related to local rarity, as an indication of extinction vulnerability. We employed generalized linear models (GLMs) to examine determinants for species richness at different scales and compared the results with those from conventional linear and breakpoint regressions. Community analyses included non‐metric multidimensional scaling, partial Mantel tests, nestedness analysis and abundance spectra. Results Regression tree analysis revealed an area threshold at 35.8 ha indicating an SIE. Tree basal area was the most important predictor of species richness on small islands (<35.8 ha). Results from GLMs supported these findings, with isolation and edge index also being important for small islands. The SIE also manifested in patterns of dung beetle community composition where communities on small islands (<35.8 ha) departed from those on the mainland and larger islands, and were highly variable with no significant nestedness, probably as a result of unexpected species occurrences on several small islands. The communities exhibited a low degree of spatial autocorrelation, suggesting that dispersal limitation plays a part in structuring dung beetle assemblages. Species with lower baseline density and an inability to forage on the forest edge were found to be rarer among sites and hence more prone to local extinction. Main conclusions We highlight the stochastic nature of dung beetle community composition on small islands and argue that this results in reduced ecosystem functionality. A better understanding of the minimum fragment size required for retaining functional ecological communities will be important for effective conservation management and the maintenance of tropical forest ecosystem stability. 相似文献
108.
109.
影响绿茶季节间品质差异的生化因子探析 总被引:3,自引:0,他引:3
通过测定同一生态环境下生长的31份茶树材料春、夏、秋3季1芽2叶时茶叶的主要生化成分,同时制作烘青茶样进行感官审评,并对测定数据进行统计分析,以揭示影响绿茶品质季节间差异的主要生化因子。结果表明,有27份材料的茶叶品质呈现出春季高于夏、秋季的变化趋势。对27份材料生化成分与品质总分的逐步回归分析、相关分析和通径分析表明,氨基酸、花青素和叶绿素含量对茶叶品质的影响最大,是导致绿茶品质季节间差异的主要生化因子。其中氨基酸表现为正向影响,在季节间呈现春季高、夏秋季降低的规律;花青素和叶绿素表现为负向影响,在季节间呈现春季低、夏秋季升高的趋势。 相似文献
110.