全文获取类型
收费全文 | 998篇 |
免费 | 69篇 |
国内免费 | 13篇 |
出版年
2024年 | 1篇 |
2023年 | 15篇 |
2022年 | 19篇 |
2021年 | 46篇 |
2020年 | 25篇 |
2019年 | 31篇 |
2018年 | 31篇 |
2017年 | 40篇 |
2016年 | 25篇 |
2015年 | 28篇 |
2014年 | 31篇 |
2013年 | 77篇 |
2012年 | 23篇 |
2011年 | 28篇 |
2010年 | 25篇 |
2009年 | 42篇 |
2008年 | 55篇 |
2007年 | 65篇 |
2006年 | 51篇 |
2005年 | 27篇 |
2004年 | 30篇 |
2003年 | 29篇 |
2002年 | 28篇 |
2001年 | 22篇 |
2000年 | 18篇 |
1999年 | 15篇 |
1998年 | 9篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 17篇 |
1994年 | 12篇 |
1993年 | 14篇 |
1992年 | 16篇 |
1991年 | 18篇 |
1990年 | 2篇 |
1989年 | 10篇 |
1988年 | 14篇 |
1987年 | 8篇 |
1986年 | 20篇 |
1985年 | 15篇 |
1984年 | 16篇 |
1983年 | 2篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 5篇 |
1979年 | 7篇 |
1978年 | 4篇 |
1977年 | 10篇 |
1976年 | 4篇 |
1974年 | 1篇 |
排序方式: 共有1080条查询结果,搜索用时 15 毫秒
221.
吉林桦甸寿山仙人洞旧石器遗址试掘报告 总被引:1,自引:4,他引:1
本文记述了吉林烨甸寿山仙人洞遗址试掘所发现的47件石制品及伴生动物化石12种.依动物化石和石器特征分析,将其地质时代暂定为晚更新世后段,考古年代为旧石器时代晚期.1991年5—6月间.吉林大学考古学系在吉林地区进行旧石器野外考古调查时,在桦甸市榆木桥子镇发现了寿山仙人洞旧石器时代遗址,并进行了试掘,在距洞口7米处开了一个横向1×3米深沟,出土了较丰富的文化遗物:47件石制品,少量的骨制品,以及大量的哺乳动物化石和一些鸟类化石. 相似文献
222.
Depth of edge influence of the agricultural-forest landscape boundary,Southwestern China 总被引:2,自引:0,他引:2
The landscape boundary is an important component of a landscape and often plays an indispensable role in regulating ecological
flows. The primary objective of this study was to estimate how far the edge effects on agricultural-forest landscape boundaries
can penetrate into the forest and agricultural field. This will serve as a basis for understanding the interaction between
forest and agricultural fields in the mountainous area of southwestern China and provide scientific basis for the practice
of the policy of “returning agricultural field to forest.” Based on field investigations, three types of boundaries with six
sampling transects were selected. We investigated the soil moisture, soil nutrients and vegetation diversity along the transect
gradient and explored the depth of edge influence (DEI) with moving split-window techniques for analyzing the data. DEI for
soil moisture changed with the seasons, ranging from 6 m in the pepper field to 2 m in the forest during the dry season, and
from 12 m in the pepper field to 2 m in the forest after heavy rain. DEI based on soil organic matter ranged from 1.5 to 10 m
in the forest field, while it was not detected based on other soil nutrient factors. DEI based on vegetation diversity varied
from 4 to 26 m and from 10 to 31 m in the forest and agricultural fields, respectively. These results provide the scientific
basis for the policy “returning agricultural field to forest”. Based on these field observations, reducing human disturbance
and revegetating with natural shrubs and meadows could be more effective for vegetation conservation in terms of soil moisture
and soil nutrient content in the arid valley of the Minjiang River. 相似文献
223.
笔石发现于辽宁辽阳兰家晚寒武世崮山组 Drepanura 带,他们为 Dendrograptus liaoyangensis sp. nov., Dendrograptus liaoyangensis latus subsp. nov., Callograptus lanjiaensis sp. nov., Callograptus sp. A, Callograptus sp. B.,这是我国已知最低层位的笔石. 相似文献
224.
225.
端刮器常见于旧石器时代晚期的细石器工业中,形制比较固定,在探讨人类行为、生计以及环境适应等方面具有重要的研究价值。国外的研究涉及制作工艺、使用方式与功能等多个方面,而国内的研究多为类型学分析,对其功能的研究相对薄弱。河北泥河湾盆地下卜庄遗址出土了一定数量的端刮器遗存,本文通过模拟打制与使用实验,结合微痕分析方法,对它们的功能进行了探索,结果表明:1)下卜庄遗址的端刮器可能主要用来加工兽皮,且加工湿皮的现象较多;2)端刮器也可能直接被用来加工木材;3)部分端刮器微痕之间的关系表明,一些端刮器在使用过程中存在对刃缘的重新修理,当它们不再适合处理兽皮时会被用来加工木材,而后废弃。 相似文献
226.
山顶洞人与现代华北人头骨非测量性特征比较及中国更新世晚期人类演化的一些问题 总被引:4,自引:1,他引:3
山顶洞人化石是迄今在东亚地区发现的数量最为丰富、保存状态最为完好的更新世晚期人类化石。多年来,山顶洞人作为东亚地区更新世晚期人类的代表,在研究东亚地区现代蒙古人种及美洲印第安人起源方面发挥着重要作用。随着对现代人起源与演化研究的深入,学术界对更新世晚期人类演化及现代人群形成过程的许多细节及演化机制予以了越来越多的注意,提出了现代人群出现时间及现代人群分化时间的概念。一些研究对山顶洞人的演化程度及其在东亚地区现代人群形成上的作用也提出了不同看法。为了深入探讨这些问题,本文对12项颅骨非测量特征在山顶洞人与现代中国人的表现情况进行了对比研究。发现有8项特征在山顶洞人与现代中国人之间具有不同的表现,作者认为山顶洞人在颅骨特征表现上较现代中国人具有更多的原始性。在此基础上,作者就更新世晚期人群内部变异、现代人群特征标准等问题进行了讨论。 相似文献
227.
Prof. Dr. Josip Tišljar M.Sc. Igor Vlahović Dr. Ivo Velić Dr. Dubravko Matičec Dr. Julie Robson 《Facies》1998,38(1):137-151
Summary During the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (presentday Istria)
specific depositional systems characterised by frequent lateral and vertical facies variations were established within a formerly
homogeneous area, ranging from peritidal and barrier bars to the offshore-transition zone. In southern Istria this period
is represented by the following succession: thin-bedded peritidal peloidal and stromatolitic limestones (Upper Albian); well-bedded
foreshore to shoreface packstones/grainstones with synsedimentary dliding and slumping (Vraconian-lowermost Cenomanian); shoreface
to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shoreface carbonate sand bodies (Lower Cenomanian);
prograding rudist bioclastic subaqueous dunes (Lower to Middle Cenomanian); rudist biostromes (Lower to Middle Cenomanian),
and high-energy rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian).
Rapid changes of depositional systems near the Albian/Cenomanian transition in Istria are mainly the result of synsedimentary
tectonics and the establishment of extensive rudist colonies producing enormous quantities of bioclastic material rather than
the influence of eustatic changes. Tectonism is evidenced by the occurrence of sliding scars, slumps, small-scale synsedimentary
faults and conspicuous bathymetric changes in formerly corresponding environments. Consequently, during the Early Cenomanian
in the region of southern Istria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment
of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were
characterised by high-energy environments with extensive rudist colonies, and high organic production leading to the progradation
of bioclastic subaqueous dunes. This resulted in numerous shallowing- and coarsening-upwards clinostratified sequences completely
infilling formerly deeper environments, and the final re-establishment of the shallow-water environments over the entire area
during the Middle Cenomanian. 相似文献
228.
229.
Jürgen Kriwet 《Pal?ontologische Zeitschrift》2002,76(1):117-123
The new pycnodont fishAnomoeodus pauciseriale is described. The single specimen consists of associated but disarticulated skeletal remains including the paired prearticulars,
some skull elements, remains of the pectoral girdle and the vertebral column. It comes from the White Chalk Formation (Upper
Cretaceous) of East Sussex near Lewes, England. The precise stratigraphic age and locality are not known. The specific identification
is based on characters of the prearticular dentitions because the other skeletal remains are not diagnostic. In contrast to
other species ofAnomoeodus, the new species is characterised by a combination of rather short prearticular dentitions with only four tooth rows, a comparable
low number of medial teeth, and the absence of intercalated teeth between tooth rows.
相似文献
230.
应用上、下解方法和抛物型方程的极值原理,研究了带存放率的竞争扩散系统ut-D1△u=u(a-bu-cv) h1vt-D2△v=v(d-eu-fv) k在齐次Neumann边界条件下解的渐近性态。 相似文献