全文获取类型
收费全文 | 62581篇 |
免费 | 5291篇 |
国内免费 | 28篇 |
出版年
2022年 | 375篇 |
2021年 | 868篇 |
2020年 | 543篇 |
2019年 | 661篇 |
2018年 | 854篇 |
2017年 | 803篇 |
2016年 | 1389篇 |
2015年 | 2441篇 |
2014年 | 2566篇 |
2013年 | 3412篇 |
2012年 | 4324篇 |
2011年 | 4343篇 |
2010年 | 2817篇 |
2009年 | 2501篇 |
2008年 | 3617篇 |
2007年 | 3656篇 |
2006年 | 3466篇 |
2005年 | 3477篇 |
2004年 | 3426篇 |
2003年 | 3226篇 |
2002年 | 3174篇 |
2001年 | 789篇 |
2000年 | 604篇 |
1999年 | 796篇 |
1998年 | 897篇 |
1997年 | 627篇 |
1996年 | 644篇 |
1995年 | 616篇 |
1994年 | 604篇 |
1993年 | 620篇 |
1992年 | 582篇 |
1991年 | 505篇 |
1990年 | 427篇 |
1989年 | 452篇 |
1988年 | 441篇 |
1987年 | 382篇 |
1986年 | 387篇 |
1985年 | 441篇 |
1984年 | 486篇 |
1983年 | 417篇 |
1982年 | 493篇 |
1981年 | 441篇 |
1980年 | 388篇 |
1979年 | 278篇 |
1978年 | 330篇 |
1977年 | 322篇 |
1976年 | 260篇 |
1975年 | 258篇 |
1974年 | 264篇 |
1973年 | 243篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The phylogeny of the fungus gnat family Mycetophilidae (Diptera) is reconstructed with a focus on the species‐rich and taxonomically difficult subfamilies Gnoristinae and Mycomyinae. The multigene phylogenetic analyses are based on five nuclear (18S, 28S, CAD, MCS, ITS2) and four mitochondrial (12S, 16S, COI, CytB) gene markers. The analyses strongly support the monophyly of Mycetophilidae and the subfamilies Manotinae, Sciophilinae, Leiinae, and Mycomyinae, although Gnoristinae is paraphyletic with respect to Mycetophilinae. All the genera and groups of genera included are supported as monophyletic, except for Acomoptera Vockeroth, Boletina Staeger, Dziedzickia Johannsen, Ectrepesthoneura Enderlein, and Neoempheria Osten Sacken. Ancestral character state reconstructions were applied to two morphological features present in Gnoristinae and Mycomyinae (i.e. presence of setae on wing membrane and wing vein R4) in order to assess their evolution. The wing vein R4 appears as an unstable character, spread throughout different clades. A dated phylogeny of the family Mycetophilidae showed that most of the subfamilies of Mycetophilidae originated and diversified during the Cretaceous. The youngest subfamilies, originated in the Paleogene, appear to be Mycomyinae and Mycetophilinae. 相似文献
2.
3.
This paper is concerned with gene survival in a population which may increase without density dependence according to a generalization of the Moran model for haploid individuals. A selective advantage to one allele and the possibility of differential reproductive rates are allowed. Simple conditions are given for ultimate homozygosity to be certain and for the possibility of ultimate polymorphism. The results complement and extend those of Heyde (1981, 1982). 相似文献
4.
5.
6.
F. H. Dawson 《Hydrobiologia》1981,77(1):71-76
The design and characteristics of inexpensive and simply constructed equal-energy response photosynthetic irradiance sensors is described for use particularly where several cells are required in comparative ecological studies either above or below water. The dimensions of the sensors can be changed proportionally to suit different applications or components. The response of the sensor to irradiance at varying angles corresponds very closely to that required by the cosine law. The sensor is comparatively insensitive to other environmental variables in field use and gave a stable output; the long term drift was proportional to electrical output but in continuous use, drift is regular and could reach -0.08 year-1 of the total. The spectral range and cosine response is discussed in comparison to other more expensive (x 5–10) commercially available, sensors and to local standards. 相似文献
7.
8.
9.
10.
Peter Buchwald 《Theoretical biology & medical modelling》2009,6(1):5-13