全文获取类型
收费全文 | 3242篇 |
免费 | 140篇 |
国内免费 | 288篇 |
专业分类
系统科学 | 262篇 |
丛书文集 | 74篇 |
教育与普及 | 44篇 |
理论与方法论 | 60篇 |
现状及发展 | 117篇 |
综合类 | 3084篇 |
自然研究 | 29篇 |
出版年
2024年 | 17篇 |
2023年 | 42篇 |
2022年 | 79篇 |
2021年 | 87篇 |
2020年 | 98篇 |
2019年 | 71篇 |
2018年 | 79篇 |
2017年 | 84篇 |
2016年 | 78篇 |
2015年 | 118篇 |
2014年 | 166篇 |
2013年 | 156篇 |
2012年 | 219篇 |
2011年 | 189篇 |
2010年 | 138篇 |
2009年 | 212篇 |
2008年 | 159篇 |
2007年 | 204篇 |
2006年 | 154篇 |
2005年 | 161篇 |
2004年 | 144篇 |
2003年 | 121篇 |
2002年 | 115篇 |
2001年 | 88篇 |
2000年 | 71篇 |
1999年 | 93篇 |
1998年 | 83篇 |
1997年 | 65篇 |
1996年 | 62篇 |
1995年 | 44篇 |
1994年 | 51篇 |
1993年 | 38篇 |
1992年 | 35篇 |
1991年 | 33篇 |
1990年 | 29篇 |
1989年 | 30篇 |
1988年 | 23篇 |
1987年 | 16篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 1篇 |
排序方式: 共有3670条查询结果,搜索用时 312 毫秒
261.
试论分子生物学在进化论发展中的作用 总被引:1,自引:0,他引:1
分子生物学是现代生物科学中最重要的分子学科。它与生物进化论有着密切的关系。本文就其在进化论发展中的作用进行综合性的阐述 相似文献
262.
Summary Enzyme electrophoretic data show a remarkably high degree of genetic similarity within the European group ofnapi s.l. whereas genetic differences exist at several loci between the European and the North American taxa ofnapi s.l. It is concluded that the European taxa did not differentiate to the species level and form a phylogenetically young group. The North American taxa included in this study are specifically distinct from Europeannapi and separated much earlier. Within these North American taxamarginalis, oleracea andvirginiensis did undergo speciation. The data show a splitting of the genusPieris into three species groups, each genetically differentiated to the same level. The splitting ofPieris into two genera, as suggested by earlier investigators, is not supported here. 相似文献
263.
张加华 《江南大学学报(自然科学版)》2000,(3)
辛亥革命后,由于政府的提倡,进步人士的倡导,物质文明和教育事业的进步及人们观念的变更,江苏社会风俗发生了巨大变化,这种风俗的变迁有其时代进步的意义,反过来,又促进了资产阶级民主革命,保存和发扬了革命成果。 相似文献
264.
地应力的作用使新洲背斜发生隆起,在浅表形成相应的劈裂、滑移及挤压区,地应力的不平衡,使辉绿岩岩浆侵入,其析出的含金、锑热液形成锑→金分带性产出,并受地层及构造控制。根据其成因分析,在新洲背斜东部核心区的周边成矿条件较为优越,存在发现新矿床的前景。 相似文献
265.
研究发展方程的周期解和反周期解的存在性, 给出了发展方程的周期与反周期的Halanay准则. 相似文献
266.
历史观是陈独秀历史哲学的主体内容。他虽无专著或专文论述历史观问题,但从散见于他的许多文章、演讲、书信中的论述来看,他的历史观,不仅脉络清晰、内容丰富,且有相当的理论深度。从纵向看,他的历史观经历了从早期进化史观向唯物史观演进的历程,具有明显的阶段性特点。 相似文献
267.
Visual pigment: G-protein-coupled receptor for light signals 总被引:5,自引:0,他引:5
The visual pigment present in photoreceptor cells is a prototypical G-protein-coupled receptor (GPCR) that receives a light
signal from the outer environment using a light-absorbing chromophore, 11-cis-retinal. Through cis-trans isomerization of the chromophore, light energy is transduced into chemical free energy, which
is in turn utilized for conformational changes in the protein to activate the retinal G-protein. In combination with site-directed
mutagenesis, various spectroscopic and biochemical studies identified functional residues responsible for chromophore binding,
color regulation, intramolecular signal transduction and G-protein coupling. Extensive studies reveal that these residues
are localized into specific domains of visual pigments, suggesting a highly manipulated molecular architecture in visual pigments.
In addition to the recent findings on dysfunctional mutations in patients with retinitis pigmentosa or congenital night blindness,
the mechanism of intramolecular signal transduction in visual pigments and their evolutionary relationship are discussed.
Received 20 July 1998; received after revision 9 September 1998; accepted 23 September 1998 相似文献
268.
269.
HE DengFa ZHOU XinYuan ZHANG ChaoJun YANG XiaoFa 《科学通报(英文版)》2007,52(A01):164-177
The Ordovician system is an important target for exploration in the Tarim Basin. Restoration of the Ordovician tectono-depositional environment is a key basis for the study of the structural development history in Tarim block and the prediction of the favorable Ordovician reservoir belts (zones). Based on the surrounding tectonic settings, the litho-facies and sedimentary fill, the palaeo-geography, palaeoclimate and palaeo-ecology, and the structural deformation and magmatic eruptions, in this paper we combine the sedimentary fill in the basin with the development of the surrounding orogenic belts and re-build the proto-type basins of the different Ordovician periods. During the Ordovician period, the Tarim region was characterized by the composite of the interior cratonic depression with the peripheral cratonic depression. The interior cratonic depression in the central and western parts is mainly the deposiUonal area of the platform facies while the peripheral cratonic depression in the eastern part is mainly the zone of deepwater basin facies, with the slope acting as the transition belt between them. During the Late Ordovician period, the closure of the Northern Kunlun Ocean and destruction of the trench-arc-basin system in the Altyn Tagh region put the southern part of the Tarim block under com- pression and uplifting and turned the central and western parts of the carbonate platform into the mixed deposiUonal continental shelf. The eastern region of the basin had the over-compensated flysch sedimentation with huge thickness, marking the drastic turn of the basin nature. The basin tectonic framework turned to the south-north differentiation of the Silurian period from the east-west differentiation of the Ordovician period. 相似文献
270.
学术界关于郁达夫的研究很多,但迄今以来少有从文学史视域对其进行探讨和研究的.而事实上,文学史中的郁达夫不仅折射出郁达夫研究的基本走向,而且也呈现了整个中国现代文学史的发展历程,是很有意义和价值的.为此从20世纪中国现代文学史研究出发,先纵向地将其分为发轫期、形成期、停滞期、转折期及沉稳期五个阶段进行史的扫描,以勾勒郁达夫在文学史上地位的变更,从侧面梳理中国现代文学史的跌宕起伏及与郁达夫研究之间的互动关系;在此基础上,再进而就影响郁达夫研究的有关关键问题发表自己的一些看法. 相似文献