全文获取类型
收费全文 | 50884篇 |
免费 | 253篇 |
国内免费 | 541篇 |
专业分类
系统科学 | 1219篇 |
丛书文集 | 122篇 |
教育与普及 | 280篇 |
理论与方法论 | 514篇 |
现状及发展 | 30485篇 |
研究方法 | 525篇 |
综合类 | 16344篇 |
自然研究 | 2189篇 |
出版年
2013年 | 799篇 |
2012年 | 513篇 |
2011年 | 2525篇 |
2009年 | 605篇 |
2008年 | 704篇 |
2007年 | 795篇 |
2006年 | 891篇 |
2005年 | 1063篇 |
2004年 | 2190篇 |
2003年 | 1800篇 |
2002年 | 1479篇 |
2001年 | 1287篇 |
2000年 | 830篇 |
1999年 | 909篇 |
1998年 | 645篇 |
1997年 | 774篇 |
1996年 | 528篇 |
1994年 | 682篇 |
1993年 | 691篇 |
1992年 | 843篇 |
1991年 | 737篇 |
1990年 | 820篇 |
1989年 | 613篇 |
1988年 | 591篇 |
1987年 | 575篇 |
1986年 | 681篇 |
1985年 | 824篇 |
1984年 | 734篇 |
1983年 | 633篇 |
1982年 | 769篇 |
1981年 | 795篇 |
1980年 | 876篇 |
1979年 | 1249篇 |
1978年 | 1150篇 |
1977年 | 1123篇 |
1976年 | 1012篇 |
1975年 | 991篇 |
1974年 | 759篇 |
1973年 | 1049篇 |
1972年 | 1135篇 |
1971年 | 1107篇 |
1970年 | 1142篇 |
1969年 | 1056篇 |
1968年 | 1034篇 |
1967年 | 875篇 |
1966年 | 762篇 |
1965年 | 627篇 |
1964年 | 465篇 |
1963年 | 414篇 |
1958年 | 435篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
This is a contribution towards a history and philosophy of modeling in its early stages in electromagnetism. In 1873, James Clerk Maxwell (1831–1879) hinted at the methodology of modeling at the end of his Treatise on Electricity and Magnetism. We focus on Maxwell's impact on physicists who immediately followed him, specifically Oliver Lodge (1851–1940) and George Francis FitzGerald (1851–1901). We begin with the role that the scientific concept of model played in the late nineteenth century, as assessed by Ludwig Boltzmann (1844–1906). We then discuss the role of hypothesis as a methodology, the appeal to (dynamical) illustration, and the way Maxwell applied model and working model in his studies of electromagnetism. We show that for Maxwell these key terms were kept distinct, but Lodge did not maintain these distinctions and, in this regard, FitzGerald followed Lodge. Notwithstanding Lodge's influence, Fitzgerald modified Maxwell's theory based on the mechanical model he designed, thereby implicitly taking the first step towards modeling. This methodology consists in drawing consequences from the (mechanical) model to the (electrodynamic) theory and modifying the latter in light of the functioning of the former. At the core of our argument is the thesis that it was a methodological novelty to move from the concept of model to the methodology of modeling. The introduction of modeling as a new methodology into physics in the late nineteenth century was a major event which deserves proper recognition. 相似文献
2.
从“十四五”保护与发展的生态关系角度,探讨了生态保护与绿色发展的十大生态关系,阐述了生态保护和绿色发展中的一系列有关生态关系的生态智慧及生态技术,并对长江、黄河等流域的生态保护和修复提出了相关建议。 相似文献
3.
R.S.K. Barnes 《Journal of Natural History》2018,52(1-2):87-113
Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as ‘Hydrobia’ knysnaensis (Krauss), ‘Assiminea’ capensis (Sowerby) and ‘Assiminea’ globulus Connolly. Although first described 80–170 years ago and present in abundance (up to 100,000 m?2), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in >85% and all three in >40% of individual 0.0026 m2 samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; ‘A.’ globulus dominating higher tidal levels, and ‘A.’ capensis and ‘Hydrobia’ lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of ‘H.’ knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of ‘A.’ globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, ‘A.’ capensis is truly aquatic. The success of these truncatelloids in unusual circumstances may be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere. 相似文献
4.
5.
采用文献资料法,依次从学理脉络、统计内容及其实施策略展开关于体育消费以及体育消费统计的讨论。认为:从学理上看,体育消费作为一个复杂概念,既牵涉宏观经济社会发展,也与文化意识形态紧密联系;而作为准确把握体育市场、把脉体育发展情况的基础性工作之一,体育消费统计则应该避免将体育消费概念泛化。应以体育政策为导向,面向人民的美好生活需要,对接国民经济统计制度,考虑建构以“体育教育与培训、体育文化与娱乐、体育保健与康复、体育传媒与信息、体育衣着耐用品、其他体育用品与服务、个人间接体育消费”为核心指标的统计框架,并通过长效工作方案的设计使体育消费统计工作制度化,通过统计调查制度的建构使体育消费统计执行有效化,通过统计数据的测算推算使体育消费统计结果精准化。 相似文献
6.
Archive for History of Exact Sciences - We show that Dedekind, in his proof of the principle of definition by mathematical recursion, used implicitly both the concept of an inductive cone from an... 相似文献
7.
Russell L. Burke Beverly Clendening Alexandra Kanonik 《Journal of Natural History》2018,52(25-26):1723-1732
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further. 相似文献
8.
9.
10.