全文获取类型
收费全文 | 204103篇 |
免费 | 13620篇 |
国内免费 | 1220篇 |
出版年
2021年 | 807篇 |
2018年 | 12781篇 |
2017年 | 11570篇 |
2016年 | 8691篇 |
2015年 | 2220篇 |
2014年 | 2304篇 |
2013年 | 2849篇 |
2012年 | 7135篇 |
2011年 | 15626篇 |
2010年 | 13840篇 |
2009年 | 10051篇 |
2008年 | 12099篇 |
2007年 | 13673篇 |
2006年 | 2665篇 |
2005年 | 2695篇 |
2004年 | 3073篇 |
2003年 | 3148篇 |
2002年 | 2878篇 |
2001年 | 9896篇 |
2000年 | 9670篇 |
1999年 | 7220篇 |
1998年 | 1629篇 |
1997年 | 1817篇 |
1996年 | 1589篇 |
1995年 | 1417篇 |
1994年 | 1313篇 |
1993年 | 1279篇 |
1992年 | 4812篇 |
1991年 | 4560篇 |
1990年 | 4000篇 |
1989年 | 4021篇 |
1988年 | 3625篇 |
1987年 | 3098篇 |
1986年 | 2777篇 |
1985年 | 2681篇 |
1984年 | 1980篇 |
1983年 | 1747篇 |
1982年 | 1233篇 |
1981年 | 984篇 |
1980年 | 916篇 |
1979年 | 1764篇 |
1978年 | 1361篇 |
1977年 | 1201篇 |
1976年 | 1028篇 |
1975年 | 1155篇 |
1974年 | 1166篇 |
1973年 | 1164篇 |
1972年 | 1279篇 |
1971年 | 1229篇 |
1970年 | 827篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
81.
82.
83.
84.
85.
86.
David Skelly 《BMC biology》2010,8(1):136
A new study of divergence in freshwater fish provides strong evidence of rapid, temperature-mediated adaptation. This study
is particularly important in the ongoing debate over the extent and significance of evolutionary response to climate change
because divergence has occurred in relatively few generations in spite of ongoing gene flow and in the aftermath of a significant
genetic bottleneck, factors that have previously been considered obstacles to evolution. Climate change may thus be more likely
to foster contemporary evolutionary responses than has been anticipated, and I argue here for the importance of investigating
their possible occurrence. 相似文献
87.
A simple and yet reliable silver impregnation method, using potassium ferrocyanide, for demonstrating nervous tissue of the rat central nervous system embedded in paraffin or paraplast is described. The method reported here is compared and discussed with earlier techniques using potassium dicyanoargentate, potassium ferrocyanide and potassium ferricyanide. 相似文献
88.
89.
Ferredoxin from Bacillus polymyxa 总被引:5,自引:0,他引:5
Y I Shethna N A Stombaugh R H Burris 《Biochemical and biophysical research communications》1971,42(6):1108-1116
90.
Winnie W.Y. Lau Marlene M. Martinez 《Journal of experimental marine biology and ecology》2003,295(1):1-21
Hydrodynamic forces can affect survival as well as limit the movement of motile benthic animals. An animal's danger of dislodgement depends on the hydrodynamic forces it experiences in its microhabitat relative to the force required to dislodge it (tenacity) from the substratum. We measured water flow and substratum characteristics in two different habitats of the shore crab Pachygrapsus crassipes: a wave-swept rocky shore and an intertidal mudflat. The maximum water velocities and accelerations in the microhabitats of the crabs at the wave-swept site were three times and two times greater, respectively, than at the mudflat site. In the laboratory, we measured the tenacity of crabs of various sizes on different substrata, and also measured their drag, lift and added-mass coefficients. Using these data, we calculated the flow conditions under which crabs would be overturned or sheared off the substratum in their two habitats. The net horizontal force (drag plus acceleration reaction) required to dislodge a crab on a rugose rock substratum was an order of magnitude greater than on smooth rock and two orders of magnitude greater than on mud. Our calculations indicate that, under non-storm conditions, crabs will not be dislodged from the substratum in either the mudflat or the wave-swept habitat when grasping the substratum with maximum tenacity. Moving crabs have lower tenacity and our calculations predict that hydrodynamic forces will restrict the mobility of large crabs more than that of small ones on smooth, but not on rugose rock. 相似文献