全文获取类型
收费全文 | 30153篇 |
免费 | 78篇 |
国内免费 | 111篇 |
专业分类
系统科学 | 119篇 |
丛书文集 | 275篇 |
教育与普及 | 89篇 |
理论与方法论 | 102篇 |
现状及发展 | 12611篇 |
研究方法 | 1168篇 |
综合类 | 15520篇 |
自然研究 | 458篇 |
出版年
2013年 | 257篇 |
2012年 | 372篇 |
2011年 | 828篇 |
2008年 | 458篇 |
2007年 | 549篇 |
2006年 | 538篇 |
2005年 | 525篇 |
2004年 | 551篇 |
2003年 | 509篇 |
2002年 | 510篇 |
2001年 | 1019篇 |
2000年 | 1017篇 |
1999年 | 597篇 |
1994年 | 343篇 |
1992年 | 532篇 |
1991年 | 452篇 |
1990年 | 507篇 |
1989年 | 501篇 |
1988年 | 482篇 |
1987年 | 483篇 |
1986年 | 519篇 |
1985年 | 594篇 |
1984年 | 450篇 |
1983年 | 446篇 |
1982年 | 375篇 |
1981年 | 364篇 |
1980年 | 414篇 |
1979年 | 947篇 |
1978年 | 753篇 |
1977年 | 687篇 |
1976年 | 576篇 |
1975年 | 673篇 |
1974年 | 901篇 |
1973年 | 739篇 |
1972年 | 747篇 |
1971年 | 973篇 |
1970年 | 1135篇 |
1969年 | 881篇 |
1968年 | 907篇 |
1967年 | 764篇 |
1966年 | 730篇 |
1965年 | 515篇 |
1964年 | 164篇 |
1959年 | 284篇 |
1958年 | 511篇 |
1957年 | 391篇 |
1956年 | 293篇 |
1955年 | 297篇 |
1954年 | 284篇 |
1948年 | 246篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Hadrons made by diquark-antidiquark pairs(tetraquarks)are held together by QCD forces in an overall color singlet.As stated in the original proposals and simila... 相似文献
2.
3.
4.
5.
6.
7.
This paper, which presents an annotated checklist of the ‘lower Nymphalidae’ (Libytheinae, Danainae, Satyrinae, Charaxinae), is the fourth in a series on the butterfly fauna of Mount Kilimanjaro. Four genera of lower Nymphalidae (Danaus, Amauris, Bicyclus, Charaxes), with a total of 11 included species, are known or believed to occur within the main forest zone, from c. 1800 to 2800 m. Of these, only three species of Charaxes (Charaxes berkeleyi, Charaxes ansorgei, Charaxes xiphares) may be restricted locally to this primary forest. The lower slopes fauna, below 1800 m, is considerably richer, with a total of 11 genera and 41 species listed (8 species of which extend into the forest zone). Possible additional species, dubious earlier records, problems with African subspecies of Danaus chrysippus, a need for more work on certain Satyrinae, and classification of the genus Charaxes are discussed. An identification key to the subfamilies of Nymphalidae, and the 19 genera of Libytheinae, Danainae, Satyrinae, Charaxinae that occur in Tanzania, together with a key to the adults of all the species of these four subfamilies considered to occur or have occurred on Kilimanjaro, with 206 colour images, are included as online Supplementary Information. 相似文献
8.
John Davenport Julia Davenport Cheong-Hoong Diong K.H. Low 《Journal of Natural History》2016,50(33-34):2097-2105
The pig-nosed freshwater turtle Carettochelys insculpta Ramsay, 1886 has paddle-shaped foreflippers that resemble those of sea turtles. These turtles exhibit a wide range of swimming capabilities. As well as swimming by the action of synchronized foreflippers alone, they sometimes used alternate hindlimb action at the same time. They could swim by ipsilaterally synchronized action of all four limbs, or by hindlimb action alone (combined with stabilizer/lift function of the foreflippers). The turtles also showed flexibility in bottom-walking. Besides the ipsilaterally synchronized quadrupedal action characteristic of other freshwater turtles, they exhibited a bipedal walking mechanism never previously described. Propelled by alternate action of the hindlimbs, the animals held the head and plastron above the substratum, with the large foreflippers acting to provide anterior lift and stability against roll and yaw. Because both hindlimbs were sometimes off the substratum simultaneously during bipedal locomotion, their duty factors were < 0.5, implying a bipedal run. 相似文献
9.
D. J. Wildish S. R. Smith T. Loeza-Quintana A. E. Radulovici S. J. Adamowicz 《Journal of Natural History》2016,50(29-30):1911-1933
Five taxa of talitrid amphipods were found in the archipelago of Bermuda, of which three were recorded there for the first time. Four of these are supralittoral wrack generalists: Platorchestia monodi BOLD:AAB3402, (a unique Molecular Operational Taxonomic Unit according to the Barcode Index Number system), a related species recognized by molecular methods, Platorchestia platensis BOLD:AAA2949, Mexorchestia carpenteri carpenteri BOLD:AAC1491 and Tethorchestia antillensis; and one a terrestrial leaf-litter generalist: Talitroides alluaudi. A key is provided to discriminate between the formally described talitrids of Bermuda. Dispersal mechanisms from the American continent to Bermuda were considered for all taxa based on species distributions along the North American Atlantic coast and also investigated by molecular methods, using genetic population differentiation and haplotype network analysis based on the barcode region of cytochrome c oxidase subunit I gene. For P. monodi BOLD:AAB3402 the genetic results suggest that some dispersal events occurred before human colonization of Bermuda but are equivocal about the source population and therefore the direction of dispersal. Some very recent synanthropic dispersal is possible with this species. For the other two species studied genetically, P. platensis BOLD:AAA2949 and M. c. carpenteri BOLD:AAC1491, the small population samples analysed support dispersal to Bermuda from the American mainland, before human occupation of Bermuda, although the available sample size was limited for these species. The available limited direct, non-genetic evidence supports synanthropic transport for Talitroides alluaudi. Platorchestia monodi BOLD:AAB3402 is found in the same wrack habitat as P. platensis BOLD:AAA2949 on Bermuda, apparently without interbreeding. No evidence was found that driftwood specialist talitrids had become established in Bermuda. 相似文献
10.
The 1980 eruption of Mount St. Helens killed trees in a broad 600-km2 swath north of the crater. Over most of the blast zone, dead trees were salvage logged and Abies procera was planted, except in areas within the Mount St. Helens National Volcanic Monument. We compared salvage-replanted sites and unsalvaged sites in 1 area of the blast zone where the sites were adjacent by using twenty-five 200-m 2 plots for each treatment. Salvaged-replanted plots had significantly lower herb and shrub cover, richness, diversity, litter depth, downed woody debris, nitrate, and phosphate. Salvaged-replanted sites also had significantly more stumps, bare area, and moss cover than unsalvaged plots. Soil organic matter and nonnative species cover did not differ. Nonnative species were not important components of any plots. Nitrate, total nitrogen, organic matter, and litter were correlated with the major patterns of species distribution in a canonical correspondence analysis of the salvaged-replanted plots. In the unsalvaged plots, slope, downed woody debris, and elevation were correlated with the major patterns of species distribution. 相似文献