全文获取类型
收费全文 | 51980篇 |
免费 | 119篇 |
国内免费 | 176篇 |
专业分类
系统科学 | 433篇 |
丛书文集 | 1111篇 |
教育与普及 | 153篇 |
理论与方法论 | 273篇 |
现状及发展 | 23842篇 |
研究方法 | 1801篇 |
综合类 | 24046篇 |
自然研究 | 616篇 |
出版年
2012年 | 610篇 |
2011年 | 1277篇 |
2010年 | 291篇 |
2008年 | 844篇 |
2007年 | 894篇 |
2006年 | 874篇 |
2005年 | 913篇 |
2004年 | 894篇 |
2003年 | 925篇 |
2002年 | 870篇 |
2001年 | 1478篇 |
2000年 | 1422篇 |
1999年 | 917篇 |
1994年 | 417篇 |
1992年 | 873篇 |
1991年 | 721篇 |
1990年 | 789篇 |
1989年 | 760篇 |
1988年 | 772篇 |
1987年 | 786篇 |
1986年 | 822篇 |
1985年 | 993篇 |
1984年 | 747篇 |
1983年 | 677篇 |
1982年 | 601篇 |
1981年 | 619篇 |
1980年 | 741篇 |
1979年 | 1592篇 |
1978年 | 1367篇 |
1977年 | 1339篇 |
1976年 | 1014篇 |
1975年 | 1113篇 |
1974年 | 1569篇 |
1973年 | 1317篇 |
1972年 | 1364篇 |
1971年 | 1714篇 |
1970年 | 2146篇 |
1969年 | 1723篇 |
1968年 | 1586篇 |
1967年 | 1605篇 |
1966年 | 1385篇 |
1965年 | 1016篇 |
1964年 | 320篇 |
1959年 | 607篇 |
1958年 | 960篇 |
1957年 | 741篇 |
1956年 | 623篇 |
1955年 | 592篇 |
1954年 | 613篇 |
1948年 | 381篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
Although theoretical studies show that overcompensatory density-dependent mechanisms can potentially generate regular or chaotic fluctuations in animal numbers, the majority of realistic single-species models of invertebrate populations are not overcompensatory enough to cause sustained population cycles. The possibility that overcompensation may generate cycles or chaos in vertebrate populations has seldom been considered. Here we show that highly overcompensatng density-dependent mortality can generate recurrent population crashes consistent with those observed in a naturally limited population of Soay sheep. The observed interval of three or more years between crashes points to sharp 'focusing' of mortality over a narrow range of population density. 相似文献
22.
Linear models are invariant under non-singular, scale-preserving linear transformations, whereas mean square forecast errors (MSFEs) are not. Different rankings may result across models or methods from choosing alternative yet isomorphic representations of a process. One approach can dominate others for comparisons in levels, yet lose to another for differences, to a second for cointegrating vectors and to a third for combinations of variables. The potential for switches in ranking is related to criticisms of the inadequacy of MSFE against encompassing criteria, which are invariant under linear transforms and entail MSFE dominance. An invariant evaluation criterion which avoids misleading outcomes is examined in a Monte Carlo study of forecasting methods. 相似文献
23.
Humoral immune reactions to heat shock proteins (hsp) from microorganisms are one aspect of microbial infections in humans. The production of antibodies which are specific to epitopes present on procaryotic hsp leads also to the appearance of cross-reactive serum antibodies in the host organism that react with human hsp. This article discusses the consequences of such autoreactive antibodies for the host in context with the development of immune tolerance and autoimmune diseases, especially rheumatoid arthritis (RA), and in experimental animal models for arthritis such as adjuvant arthritis in rats. On the basis of epitope cross-reactivity between hsp and other host proteins, a hypothesis is presented for the development of autoimmune disease following the production of hsp-specific antibodies. 相似文献
24.
Activation of a C. elegans Antennapedia homologue in migrating cells controls their direction of migration. 总被引:1,自引:0,他引:1
Anterior-posterior patterning in insects, vertebrates and nematodes involves members of conserved Antennapedia-class homeobox gene clusters (HOM-C) that are thought to give specific body regions their identities. The effects of these genes on region-specific body structures have been described extensively, particularly in Drosophila, but little is known about how HOM-C genes affect the behaviours of cells that migrate into their domains of function. In Caenorhabditis elegans, the Antennapedia-like HOM-C gene mab-5 not only specifies postembryonic fates of cells in a posterior body region, but also influences the migration of mesodermal and neural cells that move through this region. Here we show that as one neuroblast migrates into this posterior region, it switches on mab-5 gene expression; mab-5 then acts as a developmental switch to control the migratory behaviour of the neuroblast descendants. HOM-C genes can therefore not only direct region-specific patterns of cell division and differentiation, but can also act within migrating cells to programme region-specific migratory behaviour. 相似文献
25.
26.
27.
28.
29.
30.