全文获取类型
收费全文 | 23042篇 |
免费 | 2748篇 |
国内免费 | 5篇 |
专业分类
25795篇 |
出版年
2022年 | 243篇 |
2021年 | 454篇 |
2020年 | 248篇 |
2019年 | 312篇 |
2018年 | 400篇 |
2017年 | 340篇 |
2016年 | 571篇 |
2015年 | 965篇 |
2014年 | 1030篇 |
2013年 | 1184篇 |
2012年 | 1603篇 |
2011年 | 1515篇 |
2010年 | 936篇 |
2009年 | 830篇 |
2008年 | 1187篇 |
2007年 | 1218篇 |
2006年 | 1129篇 |
2005年 | 1043篇 |
2004年 | 970篇 |
2003年 | 950篇 |
2002年 | 838篇 |
2001年 | 396篇 |
2000年 | 353篇 |
1999年 | 354篇 |
1998年 | 328篇 |
1997年 | 229篇 |
1996年 | 213篇 |
1995年 | 227篇 |
1994年 | 221篇 |
1993年 | 226篇 |
1992年 | 301篇 |
1991年 | 250篇 |
1990年 | 268篇 |
1989年 | 278篇 |
1988年 | 398篇 |
1987年 | 244篇 |
1986年 | 190篇 |
1985年 | 206篇 |
1984年 | 193篇 |
1983年 | 193篇 |
1982年 | 160篇 |
1981年 | 145篇 |
1980年 | 162篇 |
1979年 | 150篇 |
1978年 | 148篇 |
1977年 | 154篇 |
1976年 | 128篇 |
1975年 | 121篇 |
1974年 | 149篇 |
1973年 | 127篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
Cascade J. B. Sorte Ron J. Etter Robert Spackman Elizabeth E. Boyle Robyn E. Hannigan 《PloS one》2013,8(11)
As the climate warms, species that cannot tolerate changing conditions will only persist if they undergo range shifts. Redistribution ability may be particularly variable for benthic marine species that disperse as pelagic larvae in ocean currents. The blue mussel, Mytilus edulis, has recently experienced a warming-related range contraction in the southeastern USA and may face limitations to northward range shifts within the Gulf of Maine where dominant coastal currents flow southward. Thus, blue mussels might be especially vulnerable to warming, and understanding dispersal patterns is crucial given the species'' relatively long planktonic larval period (>1 month). To determine whether trace elemental “fingerprints” incorporated in mussel shells could be used to identify population sources (i.e. collection locations), we assessed the geographic variation in shell chemistry of blue mussels collected from seven populations between Cape Cod, Massachusetts and northern Maine. Across this ∼500 km of coastline, we were able to successfully predict population sources for over two-thirds of juvenile individuals, with almost 80% of juveniles classified within one site of their collection location and 97% correctly classified to region. These results indicate that significant differences in elemental signatures of mussel shells exist between open-coast sites separated by ∼50 km throughout the Gulf of Maine. Our findings suggest that elemental “fingerprinting” is a promising approach for predicting redistribution potential of the blue mussel, an ecologically and economically important species in the region. 相似文献
12.
Robert Blake II Elizabeth A. Shute James Waskovsky Arthur P. Harrison JR. 《Geomicrobiology journal》2013,30(3-4):173-192
Abstract Microorganisms capable of aerobic respiration on ferrous ions are spread throughout eubacterial and archaebacterial phyla. Phylogenetically distinct organisms were shown to express spectrally distinct redox‐active biomolecules during autotrophic growth on soluble iron. A new iron‐oxidizing eubacterium, designated as strain Funis, was investigated. Strain Funis was judged to be different from other known iron‐oxidizing bacteria on the bases of comparative lipid analyses, 16S rRNA sequence analyses, and cytochrome composition studies. When grown autotrophically on ferrous ions, Funis produced conspicuous levels of a novel acid‐stable, acid‐soluble yellow cytochrome with a distinctive absorbance peak at 579 nm in the reduced state. Stopped‐flow spectrophotometric kinetic studies were conducted on respiratory chain components isolated from cell‐free extracts of Thiobacillus ferrooxidans. Experimental results were consistent with a model where the primary oxidant of ferrous ions is a highly aggregated c‐type cytochrome that then reduces the periplasmic rusticyanin. The Fe(II)‐dependent, cytochrome c‐catalyzed reduction of the rusticyanin possessed three kinetic properties in common with corresponding intact cells that respire on iron: the same anion specificity, a similar dependence of the rate on the concentration of ferrous ions, and similar rates at saturating concentrations of ferrous ions 相似文献
13.
Pollution of freshwater and estuaries 总被引:1,自引:1,他引:0
14.
15.
Elizabeth Adkins-Regan 《Behavioural processes》1985,11(2):153-158
This experiment examined the possibility that endogenous embryonic androgen contributes to sexual differentiation of behaviour in male or female quail (), and that it does so via aromatization (conversion to oestrogen). Eggs were injected on day 9 of incubation with oil or ATD (an aromatization inhibitor). As adults, males and females were exposed to short days, injected with testosterone propionate, tested for male-typical behaviour, then injected with oestradiol benzoate and tested for female-typical receptivity. ATD increased the level of male-typical copulatory behaviour in males. Male-typical behaviour in females was not affected, nor was female-typical behaviour in either sex. Thus normal male quail are actually slightly demasculinized by their own androgen during embryonic development, and this process is mediated by aromatization. 相似文献
16.
Susan Morrison Grace John-Stewart John J. Egessa Sezi Mubezi Sylvia Kusemererwa Dennis K. Bii Nulu Bulya Francis Mugume James D. Campbell Jonathan Wangisi Elizabeth A. Bukusi Connie Celum Jared M. Baeten Partners PrEP Study Team 《PloS one》2015,10(10)
During an HIV-1 prevention clinical trial in East Africa, we observed 16 cases of primary HIV-1 infection in women coincident with pregnancy or breastfeeding. Nine of eleven pregnant women initiated rapid combination antiretroviral therapy (ART), despite having CD4 counts exceeding national criteria for ART initiation; breastfeeding women initiated ART or replacement feeding. Rapid ART initiation during primary HIV-1 infection during pregnancy and breastfeeding is feasible in this setting. 相似文献
17.
Sport hunting has reportedly multiple benefits to economies and local communities; however, few of these benefits have been quantified. As part of their lease agreements with the Zambia Wildlife Authority, sport hunting operators in Zambia are required to provide annually to local communities free of charge i.e., provision a percentage of the meat obtained through sport hunting. We characterized provisioning of game meat to rural communities by the sport hunting industry in Zambia for three game management areas (GMAs) during 2004–2011. Rural communities located within GMAs where sport hunting occurred received on average > 6,000 kgs per GMA of fresh game meat annually from hunting operators. To assess hunting industry compliance, we also compared the amount of meat expected as per the lease agreements versus observed amounts of meat provisioned from three GMAs during 2007–2009. In seven of eight annual comparisons of these GMAs, provisioning of meat exceeded what was required in the lease agreements. Provisioning occurred throughout the hunting season and peaked during the end of the dry season (September–October) coincident with when rural Zambians are most likely to encounter food shortages. We extrapolated our results across all GMAs and estimated 129,771 kgs of fresh game meat provisioned annually by the sport hunting industry to rural communities in Zambia at an approximate value for the meat alone of >US$600,000 exclusive of distribution costs. During the hunting moratorium (2013–2014), this supply of meat has halted, likely adversely affecting rural communities previously reliant on this food source. Proposed alternatives to sport hunting should consider protein provisioning in addition to other benefits (e.g., employment, community pledges, anti-poaching funds) that rural Zambian communities receive from the sport hunting industry. 相似文献
18.
Jill S. Marty James P. Higham Elizabeth L. Gadsby Caroline Ross 《International journal of primatology》2009,30(6):807-823
Sexual selection has driven the evolution and elaboration of a wide variety of displays and ornaments in male nonhuman primates,
including capes, cheek flanges, and sexual coloration. Among the most sexually dimorphic of all primates is the drill (Mandrillus leucophaeus), the males of which can be 3 times the mass of females, possess large canines, and exhibit extremely bright sexual skin
coloration. However, the function of male coloration in this species has never been examined. Here, we present data on male
color (measured objectively using digital photography), dominance rank, measures of male-female association, and key sexual
behaviors, of adult male drills (n = 17) living in four semi free-ranging enclosures at the Drill Rehabilitation and Breeding Center in Nigeria. We test the
hypothesis that male coloration is a badge of status, indicating dominance rank, and the hypothesis that male coloration attracts
females. We found that male coloration did indicate rank, and that high ranking, strongly colored males were more likely to
associate with adult females, and more specifically with fully tumescent females. These males also engaged in more sexual
activity. However, measures of male-female association and sexual behaviors were not related to male color once rank had been
taken into consideration; i.e., for males of a given rank, females did not prefer those that were more colorful. We discuss
the results in light of what is known about the wild drill social system, in which unfamiliar individuals may come together
in “supergroups,” and in which quality indicators of competitive ability may be particularly important. 相似文献
19.