全文获取类型
收费全文 | 5334篇 |
免费 | 879篇 |
国内免费 | 1044篇 |
出版年
2023年 | 187篇 |
2022年 | 147篇 |
2021年 | 269篇 |
2020年 | 406篇 |
2019年 | 422篇 |
2018年 | 348篇 |
2017年 | 367篇 |
2016年 | 359篇 |
2015年 | 314篇 |
2014年 | 331篇 |
2013年 | 370篇 |
2012年 | 276篇 |
2011年 | 255篇 |
2010年 | 243篇 |
2009年 | 281篇 |
2008年 | 242篇 |
2007年 | 291篇 |
2006年 | 260篇 |
2005年 | 223篇 |
2004年 | 175篇 |
2003年 | 175篇 |
2002年 | 162篇 |
2001年 | 138篇 |
2000年 | 132篇 |
1999年 | 110篇 |
1998年 | 100篇 |
1997年 | 83篇 |
1996年 | 59篇 |
1995年 | 72篇 |
1994年 | 49篇 |
1993年 | 55篇 |
1992年 | 47篇 |
1991年 | 37篇 |
1990年 | 33篇 |
1989年 | 28篇 |
1988年 | 18篇 |
1987年 | 19篇 |
1986年 | 16篇 |
1985年 | 22篇 |
1984年 | 20篇 |
1983年 | 9篇 |
1982年 | 25篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 6篇 |
1978年 | 12篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1973年 | 8篇 |
排序方式: 共有7257条查询结果,搜索用时 125 毫秒
91.
《Chronobiology international》2013,30(5):352-360
We report a comparison of fibrinolytic variables between 10 Caucasians on a predominantly European diet and 10 Greenland Eskimos on a traditional Inuit diet containing a substantial amount of fish and sea animals. We studied the diurnal variation in tissue type plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI) antigens and activities during a 24-h period. Blood samples were taken every 4 h. The variations of the sinusoidal curves were evaluated by the Friedman χ2 test. t-PA and PAI-1 antigen in plasma fluctuated significantly during the 24 h (Eskimos p < 0.000007 and p < 0.0007; Caucasians p < 0.00003 and p < 0.02), with a peak in the early morning and a nadir in the afternoon. This also held true for PA1 activity (Eskimos p < 0.0008; Caucasians p < O.Ol), whereas t-PA activity showed an inverse but still significant pattern (Eskimos p < 0.006; Caucasians p < 0.0008). Amplitudes, areas underneath, and overall medians of the sinusoidal curves did not deviate between the two groups with respect to t-PA and PAL In contrast to the significant variation of t-PA and PAI, the plasma concentrations of fibrin degradation products (D-Dimer), a measure of effective fibrinolysis, remained constant during the 24 h, and the absolute differences between groups did not reach statistical significance. These findings suggest that circadian variation of fibrinolytic activators and inhibitors is a basic biologic phenomenon, which is not affected by life-style, dietary habits, or ethnic differences. Furthermore, the lack of diurnal variation in D-Dimer raises the question of whether there is a causal relationship between low morning activities of t-PA and the frequent onset of myocardial infarction at that time of day, as suggested by several authors. 相似文献
92.
Local climate is an important source of selection on thermal reaction norms that has been well investigated in cline studies, where populations sampled along altitudinal or latitudinal gradients are compared. Several biotic factors vary with climate, but are rarely integrated as alternative agents of selection to climatic factors. We tested the hypothesis that habitat may select for thermal reaction norms and magnitude of phenotypic plasticity in a drosophila parasitoid, independently of the climate of origin. We sampled populations of Leptopilina boulardi, a Drosophila parasitoid in two different habitats, orchards and forests. Orchards offer laying opportunities over small distances for parasitoids, with a low variability in the number of hosts per patch, while forests offer more dispersed and more variable patches. The sampling was realized in a temperate and a Mediterranean climate. We measured egg load, volume of eggs, longevity and lipid content for parasitoids reared at two temperatures. Reaction norms were opposite for populations from forests and orchards for investment in reproduction, independently of the climate of origin. The maximal investment of resources in reproduction occurred at the lower temperature in orchards and the higher temperature in forests. Host distribution differences between habitats may explain these opposite reaction norms. We also observed a flatter reaction norm for egg load in forests than in orchards. This relative canalization may have been selected in response to the higher variability in laying opportunities observed in forests. Our results demonstrate the potential role of resource distribution in evolution of thermal plasticity. 相似文献
93.
Delia Fernández Rosa María Valencia Tibor Molnár Ana Vega Eva Sagüés 《Aerobiologia》1998,14(2-3):215-220
The spores ofAlternaria andCladosporium are present throughout the year in the atmosphere of León (NW Spain), although they show an important seasonal variation.
To understand the relationship between the number of spores and climatic factors,Alternaria andCladosporium spores counts for January 1994 to December 1995 were examined by means of correlation analyses. The results of weekly samples
of both years showed that the spores concentration of two taxa are significantly and positively correlated with maximum and
minimum temperature and sunshine hours and negatively with relative humidity. The statistical analysis of daily samples generally
showed the same results. In the hourly distribution of spore concentrations we can see a similar behaviour ofAlternaria andCladosporium, with most spores collected in the 12–14 h period. 相似文献
94.
This article outlines the subsector of the information and communication technology (ICT) industry concerned with reducing the economy's environmental impact, dubbed ICT‐enabled low carbon technologies (ICTeLCTs). The article is based on a study funded by United Kingdom (UK) Trade and Investment, a division of the UK Department for Business, Innovation and Skills. ICTeLCTs can be segmented into specialist and generalist operators. Specialists focus on one or two ICT applications to monitor or reduce environmental issues, while generalists supply products and services enabling a firm or a private household to reduce the environmental impact of its activities. The subsector can be further segmented into green ICT, energy management, building management, carbon accounting, waste management, intelligent transport systems (ITSs), and water management. The main factors driving ICTeLCTs include legislation, voluntary environmental standards, corporate social responsibility (CSR) activities, customer demand, and competitive market factors. Policy makers should continue to drive the growth of ICTeLCTs with the introduction and refinement of environmental legislation regulating energy use and markets. 相似文献
95.
Reinhard Bürger Michael Lynch 《Evolution; international journal of organic evolution》1995,49(1):151-163
Because of the ubiquity of genetic variation for quantitative traits, virtually all populations have some capacity to respond evolutionarily to selective challenges. However, natural selection imposes demographic costs on a population, and if these costs are sufficiently large, the likelihood of extinction will be high. We consider how the mean time to extinction depends on selective pressures (rate and stochasticity of environmental change, and strength of selection), population parameters (carrying capacity, and reproductive capacity), and genetics (rate of polygenic mutation). We assume that in a randomly mating, finite population subject to density-dependent population growth, individual fitness is determined by a single quantitative-genetic character under Gaussian stabilizing selection with the optimum phenotype exhibiting directional change, or random fluctuations, or both. The quantitative trait is determined by a finite number of freely recombining, mutationally equivalent, additive loci. The dynamics of evolution and extinction are investigated, assuming that the population is initially under mutation-selection-drift balance. Under this model, in a directionally changing environment, the mean phenotype lags behind the optimum, but on the average evolves parallel to it. The magnitude of the lag determines the vulnerability to extinction. In finite populations, stochastic variation in the genetic variance can be quite pronounced, and bottlenecks in the genetic variance temporarily can impair the population's adaptive capacity enough to cause extinction when it would otherwise be unlikely in an effectively infinite population. We find that maximum sustainable rates of evolution or, equivalently, critical rates of environmental change, may be considerably less than 10% of a phenotypic standard deviation per generation. 相似文献
96.
Temperature responses of the cockroach, Blaberus craniifer, to rapid changes of ambient temperature (Ta) have been studied. In static conditions at Ta = 27°C the body-to-ambient temperature difference was only 0.10 ± 0.07°C. Two test situations were used, either a ramp increase of Ta from 27 to 31°C (0.1°C/min) or “step” changes from 27 to 28°C and back (0.5°C/min). In both cases body temperature closely followed Newtonian model, the body time constants measured in various conditions being very similar: 543 ± 99 sec in ramp tests, 550 ± 68 sec and 542 ± 124 sec in rising and falling step tests respectively. It is concluded that in spite of evident differences between the cockroach and an inert solid, the Newtonian model adequately represents the thermal responses of this insect to moderate changes in ambient temperature. 相似文献
97.
Kenneth C. Emberton 《Evolution; international journal of organic evolution》1995,49(3):469-475
In the southern Appalachian region of North America, the phylogenetically convergent shells of the polygyrid snails Triodopsinae Neohelix major (Binney) and Polygyrinae Mesodon normalis (Pilsbry) are even more convergent in size and shape in sympatry (7 sites) than in allopatry (23 and 10 sites). Environmental correlations account for 34% and 30% of size and shape variations in N. major (larger, taller, and more loosely coiled at northern, high-altitude, sheltered sites), but for only 14% and 9% in M. normalis (larger, flatter, and more loosely coiled at south-facing, exposed sites). The statistical significance of the sympatric convergence dropped out when these correlations were removed. This phenomenon helps account for the many cases in eastern North America of nearly identical land-snail shells in sympatry and questions the importance of competitive character displacement in the evolution of land-snail shell morphology. This apparently nonmimetic case of sympatric convergence provides an unusually precise and well-delimited, naturally replicated experiment in evolutionary morphology, which is analyzed for controlling factors in a follow-up paper. 相似文献
98.
99.
100.
G. H. Morrey 《Biometrical journal. Biometrische Zeitschrift》1989,31(5):589-598
Problems with carry-over effects in the simple two-period cross-over have lead to interest in more complex cross-over designs. A method for analysing the optimum two-treatment three-period design with binary response variables is given by making a simple extension to Gart's logistic model. The method gives independent tests for, and estimates of the difference in treatment and first-order carry-over effects. An example of the analysis is given, using the loglinear models facility in GLIM. 相似文献