全文获取类型
收费全文 | 69679篇 |
免费 | 18895篇 |
国内免费 | 1440篇 |
专业分类
90014篇 |
出版年
2024年 | 65篇 |
2023年 | 382篇 |
2022年 | 936篇 |
2021年 | 1882篇 |
2020年 | 3054篇 |
2019年 | 4681篇 |
2018年 | 4876篇 |
2017年 | 4932篇 |
2016年 | 5422篇 |
2015年 | 6276篇 |
2014年 | 6226篇 |
2013年 | 6956篇 |
2012年 | 5428篇 |
2011年 | 5116篇 |
2010年 | 5288篇 |
2009年 | 3702篇 |
2008年 | 3301篇 |
2007年 | 2768篇 |
2006年 | 2482篇 |
2005年 | 2418篇 |
2004年 | 2374篇 |
2003年 | 2074篇 |
2002年 | 1902篇 |
2001年 | 912篇 |
2000年 | 788篇 |
1999年 | 732篇 |
1998年 | 447篇 |
1997年 | 339篇 |
1996年 | 337篇 |
1995年 | 269篇 |
1994年 | 323篇 |
1993年 | 228篇 |
1992年 | 300篇 |
1991年 | 250篇 |
1990年 | 200篇 |
1989年 | 173篇 |
1988年 | 177篇 |
1987年 | 191篇 |
1986年 | 149篇 |
1985年 | 167篇 |
1984年 | 134篇 |
1983年 | 145篇 |
1982年 | 130篇 |
1981年 | 102篇 |
1980年 | 112篇 |
1979年 | 104篇 |
1978年 | 102篇 |
1977年 | 87篇 |
1976年 | 84篇 |
1974年 | 76篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
43.
44.
J. Paupério J. S. Herman J. Melo‐Ferreira M. Jaarola P. C. Alves J.B. Searle 《Molecular ecology》2012,21(24):6015-6032
Species are generally described from morphological features, but there is growing recognition of sister forms that show substantial genetic differentiation without obvious morphological variation and may therefore be considered ‘cryptic species’. Here, we investigate the field vole (Microtus agrestis), a Eurasian mammal with little apparent morphological differentiation but which, on the basis of previous sex‐linked nuclear and mitochondrial DNA (mtDNA) analyses, is subdivided into a Northern and a Southern lineage, sufficiently divergent that they may represent two cryptic species. These earlier studies also provided limited evidence for two major mtDNA lineages within Iberia. In our present study, we extend these findings through a multilocus approach. We sampled 163 individuals from 46 localities, mainly in Iberia, and sequenced seven loci, maternally, paternally and biparentally inherited. Our results show that the mtDNA lineage identified in Portugal is indeed a distinct third lineage on the basis of other markers as well. In fact, multilocus coalescent‐based methods clearly support three separate evolutionary units that may represent cryptic species: Northern, Southern and Portuguese. Divergence among these units was inferred to have occurred during the last glacial period; the Portuguese lineage split occurred first (estimated at c. 70 000 bp ), and the Northern and Southern lineages separated at around the last glacial maximum (estimated at c. 18 500 bp ). Such recent formation of evolutionary units that might be considered species has repercussions in terms of understanding evolutionary processes and the diversity of small mammals in a European context. 相似文献
45.
46.
P. Cruz C. H. Mejia‐Ruiz R. Perez‐Enriquez A. M. Ibarra 《Molecular ecology resources》2002,2(3):239-241
Five polymorphic microsatellite loci were characterized for Penaeus (Litopenaeus) vannamei. Loci were isolated using a partial Sau3A1 genomic library by the sequencing of randomly selected clones and by a biotinylated (CT)10 and (GT)10 probes screening procedure. The last strategy resulted in the most useful data. About 40% of the clones showed a previously reported satellite/microsatellite (PVS1), reducing the chance of finding new microsatellite regions. Whereas two of the microsatellite loci with more than 10 alleles will be useful for mating analysis in a breeding program, the others might prove useful for population genetic studies. 相似文献
47.
The presence of prostaglandins (PGs) was determined in gastric juice obtained from 3 conscious dogs, provided with a chronic gastric fistula. Outputs of acid (mequiv min?1) and PGs (pg min?1) were measured in gastric secretions stimulated by pentagastrin (100 or 200 ng kg?1min?1). Prostaglandin activity was estimated, after extraction and thin layer chromatography, by radioimmuno-assay of the PGB formed by treatment with alkali. Tritiated PGs were added to gastric juice for the purpose of correcting for PGs recovery. Using this method, the minimum mass of PGB which could be satisfactorily distinguished from zero was 25 pg. Prostaglandins A2 and E2 were present in pentagastrin-activated gastric secretions and averaged (mean ± SE, n = 8) 200.7 ± 18.1 and 260.1 ± 18.0 pg min?1 respectively. The identity of PGA2 and PGE2 was confirmed by gas liquid chromatography combined with mass spectrometry. The amount of PGE2 converted to PGA2 during extraction, separation and conversion procedures was estimated from the amount of [3H] PGA2 found when only [3H] PGE2 had been added to a sample of gastric juice and averaged 14.5% ± 2.0. Our preliminary results support the possibility that PGE2 and PGA2 may be of physiological importance in the regulation of canine gastric secretions. 相似文献
48.
49.
50.
Antoine Wystrach Sebastian Schwarz Patrick Schultheiss Guy Beugnon Ken Cheng 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2011,197(2):167-179
The Australian desert ant Melophorus bagoti often follows stereotypical routes through a cluttered landscape containing both distant panoramic views and obstacles (plants)
to navigate around. We created an artificial obstacle course for the ants between a feeder and their nest. Landmarks comprised
natural objects in the landscape such as logs, branches, and tussocks. Many ants travelled stereotypical routes home through
the obstacle course in training, threading repeatedly the same gaps in the landmarks. Manipulations altering the relations
between the landmarks and the surrounding panorama, however, affected the routes in two major ways. Both interchanging the
positions of landmarks (transpositions) and displacing the entire landmark set along with the starting position of the ants
(translations) (1) reduced the stereotypicality of the route, and (2) increased turns and meanders during travel. The ants
might have used the entire panorama in view-based travel, or the distal panorama might prime the identification and use of
landmarks en route. Despite the large data set, both options (not mutually exclusive) remain viable. 相似文献