全文获取类型
收费全文 | 39945篇 |
免费 | 16631篇 |
国内免费 | 12篇 |
专业分类
56588篇 |
出版年
2023年 | 76篇 |
2022年 | 219篇 |
2021年 | 643篇 |
2020年 | 2327篇 |
2019年 | 3885篇 |
2018年 | 4020篇 |
2017年 | 4302篇 |
2016年 | 4421篇 |
2015年 | 4634篇 |
2014年 | 4311篇 |
2013年 | 4890篇 |
2012年 | 2786篇 |
2011年 | 2475篇 |
2010年 | 3731篇 |
2009年 | 2398篇 |
2008年 | 1526篇 |
2007年 | 1093篇 |
2006年 | 993篇 |
2005年 | 1052篇 |
2004年 | 1006篇 |
2003年 | 959篇 |
2002年 | 920篇 |
2001年 | 388篇 |
2000年 | 285篇 |
1999年 | 298篇 |
1998年 | 210篇 |
1997年 | 145篇 |
1996年 | 150篇 |
1995年 | 135篇 |
1994年 | 130篇 |
1993年 | 122篇 |
1992年 | 114篇 |
1991年 | 97篇 |
1990年 | 115篇 |
1989年 | 93篇 |
1988年 | 91篇 |
1987年 | 91篇 |
1986年 | 79篇 |
1985年 | 83篇 |
1984年 | 98篇 |
1983年 | 87篇 |
1982年 | 102篇 |
1981年 | 90篇 |
1980年 | 94篇 |
1979年 | 74篇 |
1978年 | 70篇 |
1977年 | 62篇 |
1976年 | 73篇 |
1974年 | 52篇 |
1973年 | 65篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
103.
Bruce Y. Lee Sarah M. Bartsch Marie C. Ferguson Patrick T. Wedlock Kelly J. OShea Sheryl S. Siegmund Sarah N. Cox James A. McKinnell 《PLoS computational biology》2021,17(1)
Finding medications or vaccines that may decrease the infectious period of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) could potentially reduce transmission in the broader population. We developed a computational model of the U.S. simulating the spread of SARS-CoV-2 and the potential clinical and economic impact of reducing the infectious period duration. Simulation experiments found that reducing the average infectious period duration could avert a median of 442,852 [treating 25% of symptomatic cases, reducing by 0.5 days, reproductive number (R0) 3.5, and starting treatment when 15% of the population has been exposed] to 44.4 million SARS-CoV-2 cases (treating 75% of all infected cases, reducing by 3.5 days, R0 2.0). With R0 2.5, reducing the average infectious period duration by 0.5 days for 25% of symptomatic cases averted 1.4 million cases and 99,398 hospitalizations; increasing to 75% of symptomatic cases averted 2.8 million cases. At $500/person, treating 25% of symptomatic cases saved $209.5 billion (societal perspective). Further reducing the average infectious period duration by 3.5 days averted 7.4 million cases (treating 25% of symptomatic cases). Expanding treatment to 75% of all infected cases, including asymptomatic infections (R0 2.5), averted 35.9 million cases and 4 million hospitalizations, saving $48.8 billion (societal perspective and starting treatment after 5% of the population has been exposed). Our study quantifies the potential effects of reducing the SARS-CoV-2 infectious period duration. 相似文献
104.
105.
Distinct effects of climate warming on populations of silver fir (Abies alba) across Europe 下载免费PDF全文
106.
107.
Integrated species delimitation and conservation implications of an endangered weevil Pachyrhynchus sonani (Coleoptera: Curculionidae) in Green and Orchid Islands of Taiwan 下载免费PDF全文
YEN‐TING CHEN HUI‐YUN TSENG MING‐LUEN JENG YONG‐CHAO SU WEN‐SAN HUANG CHUNG‐PING LIN 《Systematic Entomology》2017,42(4):796-813
Oceanic islands are productive habitats for generating new species and high endemism, which is primarily due to their geographical isolation, smaller population sizes and local adaptation. However, the short divergence times and subtle morphological or ecological divergence of insular organisms may obscure species identity, so the cryptic endemism on islands may be underestimated. The endangered weevil Pachyrhynchus sonani Kôno (Coleoptera: Curculionidae: Entiminae: Pachyrhynchini) is endemic to Green Island and Orchid Island of the Taiwan‐Luzon Archipelago and displays widespread variation in coloration and host range, thus raising questions regarding its species boundaries and degree of cryptic diversity. We tested the species boundaries of P. sonani using an integrated approach that combined morphological (body size and shape, genital shape, coloration and cuticular scale), genetic (four genes and restriction site‐associated DNA sequencing, RAD‐seq) and ecological (host range and distribution) diversity. The results indicated that all the morphological datasets for male P. sonani, except for the colour spectrum, reveal overlapping but statistically significant differences between islands. In contrast, the morphology of the female P. sonani showed minimum divergence between island populations. The populations of P. sonani on the two islands were significantly different in their host ranges, and the genetic clustering and phylogenies of P. sonani established two valid evolutionary species. Integrated species delimitation combining morphological, molecular and ecological characters supported two distinct species of P. sonani from Green Island and Orchid Island. The Green Island population was described as P. jitanasaius sp.n. Chen & Lin, and it is recommended that its threatened conservation status be recognized. Our findings suggest that the inter‐island speciation of endemic organisms inhabiting both islands may be more common than previously thought, and they highlight the possibility that the cryptic diversity of small oceanic islands may still be largely underestimated. 相似文献
108.
109.
110.
Xavier Ruiz Lluis Jover Vittorio Pedrocchi Daniel Oro Jacob Gonzlez‐Sols 《Journal of avian biology》2000,31(4):567-575
During the Audouin's Gull's breeding season at the Ebro Delta in 1993, 24 fresh eggs from eight three-egg clutches (modal clutch-size) were collected at the peak of the laying period. Eggs were processed to obtain formalin-fixed yolks, which were halved and stained using the potassium dichromate method. Digitized images of the yolks were examined to assess the daily rates of yolk deposition. We used these data in combination with egg compositional analysis to build a model of energy demands during the formation of an average clutch in Audouin's Gull. To show how the different parameters of clutch formation affect the daily energy investment peak, we performed a simulation analysis in which the rapid yolk development (RYD) period, the follicle triggering interval (FTI), the laying interval (LI) and the albumen synthesis period (ASP) were allowed to vary simultaneously. In our sample, the mean RYD period was seven days with a range from six to eight days. There were no significant differences in yolk volume among eggs in a clutch, but albumen volume was significantly smaller in third eggs. According to our model the albumen synthesis of the a-egg coincides with the energy demand peak for clutch formation. This peak represents an increase by ca. 42% in female energy requirements. Values obtained from the simulation analysis showed that only the ASP of the a-egg and the RYD durations of the second and third follicles produced noticeable reductions in peak energy investment. We predict that in gulls, whose laying intervals seem to be kept constant, significant increases of the durations of the RYD periods of second and third eggs, or even significant reductions of yolk size of these eggs, may operate simultaneously to match the energy demands during clutch formation to the prevailing food conditions. 相似文献