全文获取类型
收费全文 | 10420篇 |
免费 | 900篇 |
国内免费 | 5篇 |
出版年
2022年 | 84篇 |
2021年 | 190篇 |
2020年 | 96篇 |
2019年 | 123篇 |
2018年 | 180篇 |
2017年 | 133篇 |
2016年 | 232篇 |
2015年 | 419篇 |
2014年 | 430篇 |
2013年 | 613篇 |
2012年 | 786篇 |
2011年 | 725篇 |
2010年 | 461篇 |
2009年 | 377篇 |
2008年 | 593篇 |
2007年 | 599篇 |
2006年 | 505篇 |
2005年 | 468篇 |
2004年 | 462篇 |
2003年 | 419篇 |
2002年 | 424篇 |
2001年 | 207篇 |
2000年 | 134篇 |
1999年 | 165篇 |
1998年 | 135篇 |
1997年 | 101篇 |
1996年 | 78篇 |
1995年 | 85篇 |
1994年 | 92篇 |
1993年 | 70篇 |
1992年 | 103篇 |
1991年 | 105篇 |
1990年 | 110篇 |
1989年 | 89篇 |
1988年 | 79篇 |
1987年 | 66篇 |
1986年 | 74篇 |
1985年 | 68篇 |
1984年 | 78篇 |
1983年 | 82篇 |
1982年 | 67篇 |
1981年 | 64篇 |
1980年 | 54篇 |
1979年 | 52篇 |
1978年 | 51篇 |
1977年 | 50篇 |
1976年 | 56篇 |
1975年 | 55篇 |
1974年 | 46篇 |
1973年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
Henry C. Stevens Elaine M. Metz Percy Saboya Del Castillo Juan Díaz Alvn Mark T. Bowler 《Journal of Field Ornithology》2019,90(1):70-79
White‐sand forests are patchily distributed ecosystems covering just 5% of Amazonia that host many specialist species of birds not found elsewhere, and these forests are threatened due to their small size and human exploitation of sand for construction projects. As a result, many species of birds that are white‐sand specialists are at risk of extinction, and immediate conservation action is paramount for their survival. Our objective was to evaluate current survey methods and determine the relative effect of the size of patches of these forests on the presence or absence of white‐sand specialists. Using point counts and autonomous recorders, we surveyed avian assemblages occupying patches of white‐sand forest in the Peruvian Amazon in April 2018. Overall, we detected 126 species, including 21 white‐sand forest specialists. We detected significantly more species of birds per survey point with autonomous recorders than point counts. We also found a negative relationship between avian species richness and distance from the edge of patches of white‐sand forest, but a significant, positive relationship when only counting white‐sand specialists. Although we detected more species with autonomous recorders, point counts were more effective for detecting canopy‐dwelling passerines. Therefore, we recommend that investigators conducting surveys for rare and patchily distributed species in the tropics use a mixed‐method approach that incorporates both autonomous recorders and visual observation. Finally, our results suggest that conserving large, continuous patches of white‐sand forest may increase the likelihood of survival of species of birds that are white‐sand specialists. 相似文献
3.
4.
5.
Shira Weingarten-Gabbay Susan Klaeger Siranush Sarkizova Leah R. Pearlman Da-Yuan Chen Kathleen M.E. Gallagher Matthew R. Bauer Hannah B. Taylor W. Augustine Dunn Christina Tarr John Sidney Suzanna Rachimi Hasahn L. Conway Katelin Katsis Yuntong Wang Del Leistritz-Edwards Melissa R. Durkin Christopher H. Tomkins-Tinch Pardis C. Sabeti 《Cell》2021,184(15):3962-3980.e17
- Download : Download high-res image (225KB)
- Download : Download full-size image
6.
Jin Wei Mia Madel Alfajaro Peter C. DeWeirdt Ruth E. Hanna William J. Lu-Culligan Wesley L. Cai Madison S. Strine Shang-Min Zhang Vincent R. Graziano Cameron O. Schmitz Jennifer S. Chen Madeleine C. Mankowski Renata B. Filler Neal G. Ravindra Victor Gasque Fernando J. de Miguel Ajinkya Patil Huacui Chen Craig B. Wilen 《Cell》2021,184(1):76-91.e13
- Download : Download high-res image (212KB)
- Download : Download full-size image
7.
8.
Ruth C. Paul B. Rainey Brian J. Sheehan Orla M. Keane Charles J. Dorman 《Current biology : CB》1999,9(24)
The relationship between environment and mutation is complex [1]. Claims of Lamarkian mutation [2] have proved unfounded [3], [4] and [5]; it is apparent, however, that the external environment can influence the generation of heritable variation, through either direct effects on DNA sequence [6] or DNA maintenance and copying mechanisms [7], [8], [9] and [10], or as a consequence of evolutionary processes [11], [12], [13], [14], [15] and [16]. The spectrum of mutational events subject to environmental influence is unknown [6] and precisely how environmental signals modulate mutation is unclear. Evidence from bacteria suggests that a transient recombination-dependent hypermutational state can be induced by starvation [5]. It is also apparent that chnages in the mutability of specific loci can be influenced by alterations in DNA topology [10] and [17]. Here we describe a remarkable instance of adaptive evolution in Salmonella which is caused by a mutation that occurs in intermediate-strength osmotic environments. We show that the mutation is not ‘directed’ and describe its genetic basis. We also present compelling evidence in support of the hypothesis that the mutational event is constrained by signals transmitted from the external environment via changes in the activity of DNA gyrase. 相似文献
9.
10.