全文获取类型
收费全文 | 4311篇 |
免费 | 575篇 |
国内免费 | 383篇 |
出版年
2024年 | 16篇 |
2023年 | 88篇 |
2022年 | 117篇 |
2021年 | 163篇 |
2020年 | 184篇 |
2019年 | 246篇 |
2018年 | 164篇 |
2017年 | 236篇 |
2016年 | 213篇 |
2015年 | 211篇 |
2014年 | 244篇 |
2013年 | 341篇 |
2012年 | 208篇 |
2011年 | 218篇 |
2010年 | 187篇 |
2009年 | 241篇 |
2008年 | 233篇 |
2007年 | 229篇 |
2006年 | 238篇 |
2005年 | 191篇 |
2004年 | 145篇 |
2003年 | 139篇 |
2002年 | 121篇 |
2001年 | 108篇 |
2000年 | 72篇 |
1999年 | 75篇 |
1998年 | 73篇 |
1997年 | 53篇 |
1996年 | 53篇 |
1995年 | 60篇 |
1994年 | 47篇 |
1993年 | 36篇 |
1992年 | 35篇 |
1991年 | 35篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 21篇 |
1987年 | 26篇 |
1986年 | 26篇 |
1985年 | 20篇 |
1984年 | 26篇 |
1983年 | 16篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 7篇 |
1976年 | 4篇 |
1973年 | 2篇 |
排序方式: 共有5269条查询结果,搜索用时 328 毫秒
31.
32.
中更新世以来兽类地理分布的变化及其天文气候学的解释 总被引:5,自引:0,他引:5
中更新世以来,兽类的地理分布曾发生过重大的变化,如西伯利亚的兽类曾三次大举西迁,入侵西欧.它们分别发生在氧同位素的第16、6、2-4阶段.如果考察过去73万年以来全球所获得的、北半球冬半年平均的太阳辐射量的变化,那么第16、6、2-4阶段乃是太阳辐射量比较低的时期.故中更新世以来兽类地理分布的变化是受气候变迁的制约的.而后者则是受天文因素的自然变化所控制的. 相似文献
33.
The Castel di Guido site, with an estimated age of approximately 300,000 years, has yielded abundant animal remains, Acheulian stone and bone bifaces, and small tools. On the surface of the original deposit, turned over by agricultural activities, fragments of human remains were discovered between 1980 and 1986, including a temporal bone described here. 相似文献
34.
The Streptomyces plasmid SCP2*: its functional analysis and development into useful cloning vectors 总被引:43,自引:0,他引:43
Detailed restriction maps of the plasmid SCP2* and its deletion derivative pSCP103 were constructed. DNA fragments carrying hygromycin (Hyg), thiostrepton (Thio) or viomycin-resistance (VioR) determinants were inserted into pSCP103, and various segments were deleted from the resulting plasmids. Changes in plasmid phenotypes associated with these insertions and deletions allowed the localisation and characterisation of plasmid replication, stability, transfer and fertility functions. Several useful cloning vectors were constructed. They are able to maintain large (greater than 30 kb) DNA inserts, with stable inheritance at a low copy number (1-2 per chromosome) and without structural rearrangements, in Streptomyces hosts. The vectors have a broad host range in the genus Streptomyces. One of them (pIJ903) is a shuttle vector for Streptomyces and Escherichia coli. 相似文献
35.
36.
青海贵德、共和两盆地晚新生代哺乳动物 总被引:22,自引:3,他引:19
本文记述了青海省贵德及共和两盆地的几个含晚新生代哺乳动物化石地点的地层剖面及采自18个地点的26种(代表20个属)哺乳动物化石.对其中的 Myospalax arvicolinus, Microtus, Anancus, Leptobos crassus gonghenensis subsp. nov., L. vallisarni, Bison (Superbison) crassicornis, Boopsis breviceps作了较详细的描述和讨论.通过对动物群的分析初步确定了盆地主要堆积物的时代——贵德盆地:上新世—早更新世;共和盆地:早—中更新世,并简略探讨了两盆地的发育历史. 相似文献
37.
38.
39.
Phytoplankton ecology in an unusually stable environment (Montezuma Well,Arizona, U.S.A.) 总被引:1,自引:0,他引:1
Relatively minor annual amplitudes of change in certain major nutrients, and especially pH and water temperature were measured in the spring-fed system of Montezuma Well, Arizona during a four year study. phytoplankton diversity was low but for the most part, composition was spatially and temporally constant; total seasonal phytoplankton density was significantly correlated with regional incident light. Phytoplankton species composition changed briefly during and for a short period following the summer monsoon. Ultraplankton (<5 µm diam.) numerically comprised nearly 80% of the phytoplankton community throughout most of the year. The limited residence time of water in the Well may have provided a competitive advantage for cells with high surface area:volume ratios and correspondingly rapid division rates. Nannochloris bacillaris Naum. and Coccomyxa minor Skuja were perennial dominants. Diatom populations did not increase with annual increases in vernal solar radiation. Low pH, high dissolved CO2, and limited residence time for metabolic inhibitors are considered to be largely responsible for the reduced blue-green populations in the Well. The only flagellated photosynthetic group present in Montezuma Well was the Cryptophyta. Desmid populations were minimal, even though pH was consistently below circumneutral (6.5) and free CO2 concentrations high. The role of grazing by an amphipod, Hyalella montezuma, on annual phytoplankton abundance is examined. 相似文献
40.
Enamel hypoplasia was recorded for the primary and permanent teeth of Natufians (terminal Pleistocene Hunter-Gatherers) and
Early Arabs (Subsistence farmers). Minimal hypoplasia was found in the primary teeth of the Natufians, but in the Early Arab
population, 31% of upper first primary molars showed enamel defects. In the permanent teeth, the Early Arab population again
showed a significantly higher frequency of hypoplasia, with all teeth affected. In the Natufians, only later developing teeth
showed enamel defects. These findings suggest that health status in the Early Arab population was poor throughout life, affecting
pregnant mothers, their foetuses, young infants and children. These results confirm other studies that have shown deterioration
in health status of early agriculturalists relative to hunter- gatherers. Comparisons of these findings with those of recent
populations indicate that health status in the Natufians was comparable to that of modern populations with subsistence diets
and mediocre health care. Health status in the Early Arab population was comparable to that of modern populations suffering
from severe malnutrition and chronic disease. 相似文献