全文获取类型
收费全文 | 6188篇 |
免费 | 613篇 |
国内免费 | 9篇 |
专业分类
6810篇 |
出版年
2023年 | 25篇 |
2022年 | 56篇 |
2021年 | 91篇 |
2020年 | 79篇 |
2019年 | 70篇 |
2018年 | 95篇 |
2017年 | 78篇 |
2016年 | 114篇 |
2015年 | 209篇 |
2014年 | 240篇 |
2013年 | 291篇 |
2012年 | 362篇 |
2011年 | 408篇 |
2010年 | 264篇 |
2009年 | 210篇 |
2008年 | 319篇 |
2007年 | 357篇 |
2006年 | 340篇 |
2005年 | 340篇 |
2004年 | 292篇 |
2003年 | 269篇 |
2002年 | 295篇 |
2001年 | 75篇 |
2000年 | 81篇 |
1999年 | 99篇 |
1998年 | 87篇 |
1997年 | 59篇 |
1996年 | 48篇 |
1995年 | 59篇 |
1994年 | 49篇 |
1993年 | 46篇 |
1992年 | 53篇 |
1991年 | 60篇 |
1990年 | 58篇 |
1989年 | 52篇 |
1988年 | 46篇 |
1987年 | 37篇 |
1986年 | 34篇 |
1985年 | 46篇 |
1984年 | 34篇 |
1983年 | 43篇 |
1982年 | 46篇 |
1981年 | 52篇 |
1980年 | 51篇 |
1979年 | 40篇 |
1978年 | 32篇 |
1976年 | 41篇 |
1975年 | 37篇 |
1974年 | 25篇 |
1973年 | 27篇 |
排序方式: 共有6810条查询结果,搜索用时 15 毫秒
51.
Innominato PF Giacchetti S Moreau T Smaaland R Focan C Bjarnason GA Garufi C Iacobelli S Tampellini M Tumolo S Carvalho C Karaboué A Lévi F;ARTBC International Chronotherapy Group 《Chronobiology international》2011,28(7):586-600
Circadian clocks control cellular proliferation and drug metabolism over the 24?h. However, circadian chronomodulated chemotherapy with 5-fluorouracil, leucovorin, and oxaliplatin (chronoFLO4) offered no survival benefit as compared with the non-time-stipulated FOLFOX2, in an international randomized trial involving patients with previously untreated metastatic colorectal cancer (EORTC 05963). The authors hypothesized that treatment near maximum tolerated dose could disrupt circadian clocks thus impairing the efficacy of chronoFLO4 but not of FOLFOX2. Patients with available data (N?=?556) were categorized into three subgroups according to the worst grade (G) of neutropenia experienced during treatment. Distinct multivariate models with time-dependent covariates were constructed for each treatment schedule. Neutropenia incidence (all grades) was 33% on chronoFLO4 and 61% on FOLFOX2 (p?.0001), and G3-4 were 7% and 25%, respectively (p < .0001). Neutropenia was significantly more frequent in women than men on either schedule (FOLFOX2, p = .003; chronoFLO4, p = .04). Median survival was 20.7 mo in patients with G3-4 neutropenia versus 12.5 mo in neutropenia-free patients on FOLFOX2 (p < .0001). Corresponding figures were 13.7 and 19.4 mo, respectively, on chronoFLO4 (p?=?.36). Multivariate analysis confirmed occurrence of severe neutropenia independently predicted for better overall survival on FOLFOX2 (HR?=?0.56; p = .015), and worse survival on chronoFLO4 (HR?=?1.77, p = .06), with a significant interaction test (p < .0001). Prediction of better survival in neutropenic patients on FOLFOX2 supports the administration of conventional chemotherapy near maximum tolerated dose. The opposite trend shown here for chronoFLO4 supports the novel concept of jointly optimized hematologic tolerability and efficacy through personalized circadian-timed therapy. 相似文献
52.
53.
ABSTRACT Point count surveys are widely used for monitoring songbird populations, but little is known of the effect of the observer on songbird behavior during point counts. We used a novel, wireless array of recorders to determine the location of singing birds with and without the presence of an observer. The array consisted of seven autonomous recording units synchronized to Global Positioning System (GPS) clocks, set around the perimeter of a 50‐m‐radius circle with one in the middle. Units were set to record automatically from half an hour before dawn until 10:00 each morning. We sampled 26 different locations in old fields at the Prince Edward Point National Wildlife Area in eastern Ontario between 1 June and 4 July 2007. Position estimates derived from a time‐lag cross‐correlation algorithm had a mean error of 1.7 m within 50 m and 5.6 m at 100 m from the center of the array. We found no difference in the positions of birds when an observer was present or absent. We also found no difference in the number of individuals or species detected or in the onset of singing. Our results suggest that, at least in the community we studied, observers conducting point counts do not cause significant changes in bird behavior. 相似文献
54.
55.
56.
Shane Gero Marina Milligan Caroline Rinaldi Pernell Francis Jonathan Gordon Carole Carlson Andrea Steffen Peter Tyack Peter Evans Hal Whitehead 《Marine Mammal Science》2014,30(3):905-922
There is substantial geographic variation in the behavior and social structure of sperm whales worldwide. The population in the Eastern Caribbean is thought to be isolated from other areas in the North Atlantic. We describe the behavior and social structure of the sperm whales identified off Dominica during an eight year study (2005–2012; 92% of photographic identifications) with supplementary data collected from seven other organizations dating as far back as 1981. A total of 419 individuals were identified. Resighting rates (42% of individuals between years) and encounter rates with sperm whale groups (mean = 80.4% of days at sea) among this population were both comparatively high. Group sizes were small (7–9 individuals) and were comprised of just one social unit (mean = 6.76 individuals, SD = 2.80). We described 17 units which have been reidentified off Dominica across 2–27 yr. Mature males are seen regularly off Dominica, but residency in the area lasts only a few days to a few weeks. Males were reidentified across years spanning up to a decade. Management of this population within the multinational Wider Caribbean Region will require governments to work towards international agreements governing sperm whales as a cross‐border species of concern. 相似文献
57.
Insect workers can increase their inclusive fitness by biasing colony sex allocation towards males when their mother queen is mated to multiple males and females when she is singly mated. Workers need heritable variation in odour diversity to assess queen mating frequency. Here we present a simple one-locus two-allele model, which shows that the sex ratio specialization itself will often select against rare alleles that would provide additional information for the assessment of queen mating frequency. However, under certain rather restricted conditions, such as when sex ratios are highly female biased, and when worker reproduction is rare, sex ratio specialization can select for rare alleles. This suggests that sex allocation biasing by workers will usually reduce the very information that workers need to assess queen mating frequency. Our model is an example where an explicit treatment of underlying genetics and mechanisms of behaviour, such as information use, is necessary to fully understand the evolution of an adaptive behavioural strategy. 相似文献
58.
59.
60.
Nest-mate recognition template of guard honeybees (Apis mellifera) is modified by wax comb transfer 下载免费PDF全文
Couvillon MJ Caple JP Endsor SL Kärcher M Russell TE Storey DE Ratnieks FL 《Biology letters》2007,3(3):228-230
In recognition, discriminators use sensory information to make decisions. For example, honeybee (Apis mellifera) entrance guards discriminate between nest-mates and intruders by comparing their odours with a template of the colony odour. Comb wax plays a major role in honeybee recognition. We measured the rejection rates of nest-mate and non-nest-mate worker bees by entrance guards before and after a unidirectional transfer of wax comb from a 'comb donor' hive to a 'comb receiver' hive. Our results showed a significant effect that occurred in one direction. Guards in the comb receiver hive became more accepting of non-nest-mates from the comb donor hive (rejection decreased from 70 to 47%); however, guards in the comb donor hive did not become more accepting of bees from the comb receiver hive. These data strongly support the hypothesis that the transfer of wax comb increases the acceptance of non-nest-mates not by changing the odour of the bees, but by changing the template used by guards. 相似文献