全文获取类型
收费全文 | 759917篇 |
免费 | 86785篇 |
国内免费 | 374篇 |
专业分类
847076篇 |
出版年
2016年 | 8392篇 |
2015年 | 11556篇 |
2014年 | 13522篇 |
2013年 | 20013篇 |
2012年 | 21856篇 |
2011年 | 22488篇 |
2010年 | 15273篇 |
2009年 | 14085篇 |
2008年 | 19980篇 |
2007年 | 20884篇 |
2006年 | 19568篇 |
2005年 | 18802篇 |
2004年 | 18547篇 |
2003年 | 18202篇 |
2002年 | 17729篇 |
2001年 | 32587篇 |
2000年 | 32833篇 |
1999年 | 26214篇 |
1998年 | 9433篇 |
1997年 | 10181篇 |
1996年 | 9591篇 |
1995年 | 9052篇 |
1994年 | 8812篇 |
1993年 | 9011篇 |
1992年 | 22707篇 |
1991年 | 22455篇 |
1990年 | 21916篇 |
1989年 | 21628篇 |
1988年 | 20051篇 |
1987年 | 19141篇 |
1986年 | 17854篇 |
1985年 | 18308篇 |
1984年 | 15266篇 |
1983年 | 13180篇 |
1982年 | 10193篇 |
1981年 | 9103篇 |
1980年 | 8677篇 |
1979年 | 14607篇 |
1978年 | 11470篇 |
1977年 | 10675篇 |
1976年 | 10178篇 |
1975年 | 11069篇 |
1974年 | 11810篇 |
1973年 | 11668篇 |
1972年 | 10727篇 |
1971年 | 9759篇 |
1970年 | 8330篇 |
1969年 | 8148篇 |
1968年 | 7467篇 |
1967年 | 6555篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
943.
944.
In this paper we describe the use of punched feature cards in a general practice for 18 months. Its advantages are the low cost, speed of information retrieval, visible statistics, computer compatibility, accuracy, confidentiality, flexibility, and simplicity of setting up and collection of information. The system encourages the doctor to ask questions about his practice, and could readily be adopted in other practices. 相似文献
945.
Ho WW Fernandes CC Alves-Gomes JA Smith GT 《Ethology : formerly Zeitschrift fur Tierpsychologie》2010,116(11):1050-1064
The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior. 相似文献
946.
Divergence time estimates of mammals from molecular clocks and fossils: Relevance of new fossil finds from India 总被引:1,自引:0,他引:1
G. V. R. Prasad 《Journal of biosciences》2009,34(5):649-659
This paper presents a brief review of recent advances in the classification of mammals at higher levels using fossils and
molecular clocks. It also discusses latest fossil discoveries from the Cretaceous — Eocene (66–55 m.y.) rocks of India and
their relevance to our current understanding of placental mammal origins and diversifications. 相似文献
947.
948.
The Capricorn silvereye (Zosterops lateralis chlorocephalus) is ideally suited to investigating the genetic basis of body size evolution. We have isolated and characterized a set of microsatellite markers for this species. Seven out of 11 loci were polymorphic. The number of alleles detected ranged from two to five and observed heterozygosities between 0.12 and 0.67. One locus, ZL49, was found to be sex‐linked. This moderate level of diversity is consistent with that expected in an isolated, island population. 相似文献
949.
D. L. Grekov 《Plasma Physics Reports》2001,27(12):1011-1016
A possible way to affect the influxes of heavy impurities into the plasma of a stellarator with poloidal slits in the vacuum wall (e.g., the W7-AS stellarator) by RF heating of the impurities is discussed. It is shown that the influxes can be reversed in direction by applying a relatively low RF power. The design features of the W7-AS stellarator are such that there is no need to place additional antennas inside the vacuum chamber. 相似文献
950.