全文获取类型
收费全文 | 11978篇 |
免费 | 47篇 |
国内免费 | 72篇 |
专业分类
系统科学 | 67篇 |
丛书文集 | 30篇 |
教育与普及 | 21篇 |
理论与方法论 | 33篇 |
现状及发展 | 5482篇 |
研究方法 | 653篇 |
综合类 | 5664篇 |
自然研究 | 147篇 |
出版年
2012年 | 236篇 |
2011年 | 346篇 |
2010年 | 89篇 |
2009年 | 76篇 |
2008年 | 235篇 |
2007年 | 281篇 |
2006年 | 277篇 |
2005年 | 256篇 |
2004年 | 225篇 |
2003年 | 221篇 |
2002年 | 275篇 |
2001年 | 435篇 |
2000年 | 432篇 |
1999年 | 292篇 |
1992年 | 237篇 |
1991年 | 194篇 |
1990年 | 217篇 |
1989年 | 193篇 |
1988年 | 202篇 |
1987年 | 219篇 |
1986年 | 190篇 |
1985年 | 252篇 |
1984年 | 227篇 |
1983年 | 171篇 |
1982年 | 176篇 |
1981年 | 180篇 |
1980年 | 170篇 |
1979年 | 405篇 |
1978年 | 341篇 |
1977年 | 252篇 |
1976年 | 293篇 |
1975年 | 260篇 |
1974年 | 272篇 |
1973年 | 226篇 |
1972年 | 244篇 |
1971年 | 308篇 |
1970年 | 374篇 |
1969年 | 254篇 |
1968年 | 307篇 |
1967年 | 292篇 |
1966年 | 249篇 |
1965年 | 181篇 |
1964年 | 101篇 |
1959年 | 88篇 |
1958年 | 162篇 |
1957年 | 100篇 |
1956年 | 91篇 |
1955年 | 85篇 |
1954年 | 77篇 |
1948年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
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
2.
This paper undertakes a comprehensive examination of 10 measures of core inflation and evaluates which measure produces the best forecast of headline inflation out‐of‐sample. We use the Personal Consumption Expenditure Price Index as our measure of inflation. We use two sets of components (17 and 50) of the Personal Consumption Expenditure Price Index to construct these core inflation measures and evaluate these measures at the three time horizons (6, 12 and 24 months) most relevant for monetary policy decisions. The best measure of core inflation for both sets of components and over all time horizons uses weights based on the first principal component of the disaggregated (component‐level) prices. Interestingly, the results vary by the number of components used; when more components are used the weights based on the persistence of each component is statistically equivalent to the weights generated by the first principal component. However, those forecasts using the persistence of 50 components are statistically worse than those generated using the first principal component of 17 components. The statistical superiority of the principal component method is due to the fact that it extracts (in the first principal component) the common source of variation in the component level prices that accurately describes trend inflation over the next 6–24 months. 相似文献
3.
4.
Brian Young 《Studies in history and philosophy of science》2004,35(3):645-663
The career of John Jackson (1686-1763), Arian theologian and controversialist, provides a key to unlocking the early reception and quick collapse of a Newtonian natural apologetic originally developed by Samuel Clarke. The importance of friendship and discipleship in eighteenth-century intellectual enquiry is emphasised, and the links between Newton and his followers are traced alongside those of a group of Cambridge Lockeans, led by Jackson’s direct contemporary Daniel Waterland, who proved instrumental in the initial dismantling of Clarke’s brand of Newtonian apologetic. The controversial context of this engagement is shown to have been largely provided by the religiously compromising rise of freethinking, and Tindal’s Christianity as old as the creation (1731) signalled the dangers to proponents of natural religion as an adjunct of Christian apologetic in such a heated atmosphere. Religious division of the sort that resulted paradoxically played into the hands of the freethinkers in the anticlerical atmosphere of the 1730s, and accusations were exchanged between Newtonians and Lockeans accordingly. The dynamic of England’s Enlightenment experience is, then, a complicated one, and, as the career and writings of Jackson and William Whiston demonstrate, it was one which absorbed as well as repudiated ‘enthusiasm’. 相似文献
5.
6.
7.
8.
9.
Total Systems Intervention (TSI) is an approach to intervening in problem situations which has much to offer where complex
interacting issues need to be addressed by the complementary use of intervention methodologies. That such an approach has
much in common with Action Research (AR) has been recognized, with much recent effort being devoted to the relationship between
AR and Critical Systems Thinking (CST), the theoretical endeavor underpinning TSI. This paper further develops this line of
debate and relates AR or Human Inquiry (HI) more directly to TSI, using an information systems intervention to enhance the
study. The outcome is a demonstration of how TSI implicitly uses techniques informed from the field of Action Research, and
how a more thorough synthesis of HI with TSI might serve to improve the overall intervention process. 相似文献
10.
Vitamin B12 (methylcobalamin) was administered orally (3 mg/day) to 9 healthy subjects for 4 weeks. Nocturnal melatonin levels after exposure to bright light (ca. 2500 lx) were determined, as well as the levels of plasma melatonin over 24 h. The timing of sleep was also recorded. Vitamin B12 was given blind to the subjects and crossed over with placebo. We found that the 24-h melatonin rhythm was significantly phase-advanced (1.1 h) in the vitamin B12 trial as compared with that in the placebo trial. In addition, the 24-h mean of plasma melatonin level was much lower in the vitamin B12 trial than with the placebo. Furthermore, the nocturnal melatonin levels during bright light exposure were significantly lower in the vitamin B12 trial than with the placebo. On the other hand, vitamin B12 did not affect the timing of sleep. These findings raise the possibility that vitamin B12 phase-advances the human circadian rhythm by increasing the light sensitivity of the circadian clock. 相似文献