全文获取类型
收费全文 | 686761篇 |
免费 | 67144篇 |
国内免费 | 399篇 |
出版年
2018年 | 8097篇 |
2017年 | 7917篇 |
2016年 | 11842篇 |
2015年 | 17295篇 |
2014年 | 18769篇 |
2013年 | 24747篇 |
2012年 | 27309篇 |
2011年 | 25382篇 |
2010年 | 17285篇 |
2009年 | 15279篇 |
2008年 | 20825篇 |
2007年 | 21338篇 |
2006年 | 19882篇 |
2005年 | 24345篇 |
2004年 | 22793篇 |
2003年 | 20316篇 |
2002年 | 17342篇 |
2001年 | 30577篇 |
2000年 | 29923篇 |
1999年 | 24240篇 |
1998年 | 7604篇 |
1997年 | 7916篇 |
1996年 | 7259篇 |
1995年 | 6833篇 |
1994年 | 6582篇 |
1993年 | 6412篇 |
1992年 | 19343篇 |
1991年 | 18595篇 |
1990年 | 18010篇 |
1989年 | 17444篇 |
1988年 | 16065篇 |
1987年 | 14865篇 |
1986年 | 13787篇 |
1985年 | 13679篇 |
1984年 | 11030篇 |
1983年 | 9193篇 |
1982年 | 6694篇 |
1981年 | 6071篇 |
1980年 | 5687篇 |
1979年 | 10259篇 |
1978年 | 7777篇 |
1977年 | 7085篇 |
1976年 | 6472篇 |
1975年 | 7260篇 |
1974年 | 7915篇 |
1973年 | 7720篇 |
1972年 | 6991篇 |
1971年 | 6480篇 |
1970年 | 5598篇 |
1969年 | 5351篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
951.
952.
OBJECTIVE: To determine the vaccination rate among infants discharged from a neonatal intensive care unit (NICU) and factors affecting that rate. DESIGN: Cross-sectional survey conducted when the children were 12 to 18 months of age. SETTING: NICU at the Royal University Hospital, Saskatoon, Sask. PARTICIPANTS: All 395 infants discharged from the NICU between Jan. 1 and June 30, 1992. MAIN OUTCOME MEASURES: Vaccination rate, ethnic background (native or non-native), place of residence (urban or rural), health status (number of days spent in the NICU), reasons for delay in or incomplete vaccinations (those involving parents'' responsibility, infant illness or contraindications). RESULTS: Of the 395 infants, 20 (5.0%) had died and incomplete information was available for 30 (7.6%). Complete data were available for 345 (87.3%). Of the infants for whom data were available, 8 (2.3%) had never been vaccinated and 142 (41.2%) had a delayed vaccination schedule or had not completed their scheduled vaccinations. Only 195 (56.6%) of the infants had received a full vaccination series. Non-native ethnic background was a predictor of completed vaccinations (odds ratio [OR] 5.40, 95% confidence interval [CI] 3.05 to 9.52). In a univariate model, urban area of residence was not a significant predictor of vaccination status, but when ethnic background was controlled for in a multivariate logistic regression analysis, urban area of residence was found to be inversely associated with completed vaccinations (OR 0.34, 95% CI 0.15 to 0.79). The number of days the child had spent in the NICU was not a significant predictor of vaccination status. CONCLUSION: The vaccination rate of infants discharged from the NICU is not optimal. Urban native children appears to be at risk of not being vaccinated. Non-native infants are five times more likely than native infants to have completed all of their scheduled vaccinations. Methods to improve the rate of completed vaccinations, especially for native children, must be sought and tested. 相似文献
953.
A sucrase gene from Leuconostoc mesenteroides was cloned and expressed in Escherichia coli. The cloned enzyme did not show dextransucrase or sucrose phosphorylase activity. HPLC and GC-MS analyses of the sucrase products indicated the presence of fructose and glucose in equimolar amounts. IPTG induction did not increase sucrase activity in E. coli indicating that the cloned gene may be transcribed from its own promoter. To our knowledge, this is the first sucrase cloned from L. mesenteroides that has invertase activity. 相似文献
954.
Valeria L. Martin-Albarracin Guillermo C. Amico Daniel Simberloff Martin A. Nu?ez 《PloS one》2015,10(11)
Introduction and naturalization of non-native species is one of the most important threats to global biodiversity. Birds have been widely introduced worldwide, but their impacts on populations, communities, and ecosystems have not received as much attention as those of other groups. This work is a global synthesis of the impact of nonnative birds on native ecosystems to determine (1) what groups, impacts, and locations have been best studied; (2) which taxonomic groups and which impacts have greatest effects on ecosystems, (3) how important are bird impacts at the community and ecosystem levels, and (4) what are the known benefits of nonnative birds to natural ecosystems. We conducted an extensive literature search that yielded 148 articles covering 39 species belonging to 18 families -18% of all known naturalized species. Studies were classified according to where they were conducted: Africa, Asia, Australasia, Europe, North America, South America, Islands of the Indian, of the Pacific, and of the Atlantic Ocean. Seven types of impact on native ecosystems were evaluated: competition, disease transmission, chemical, physical, or structural impact on ecosystem, grazing/ herbivory/ browsing, hybridization, predation, and interaction with other non-native species. Hybridization and disease transmission were the most important impacts, affecting the population and community levels. Ecosystem-level impacts, such as structural and chemical impacts were detected. Seven species were found to have positive impacts aside from negative ones. We provide suggestions for future studies focused on mechanisms of impact, regions, and understudied taxonomic groups. 相似文献
955.
956.
We combine information about the evolutionary history and distributional patterns of the genus Saintpaulia H. Wendl. (Gesneriaceae; ‘African violets’) to elucidate the factors and processes behind the accumulation of species in tropical montane areas of high biodiversity concentration. We find that high levels of biodiversity in the Eastern Arc Mountains are the result of pre-Quaternary speciation processes and environmental stability. Our results support the hypothesis that climatically stable mountaintops may have acted as climatic refugia for lowland lineages during the Pleistocene by preventing extinctions. In addition, we found evidence for the existence of lowland micro-refugia during the Pleistocene, which may explain the high species diversity of East African coastal forests. We discuss the conservation implications of the results in the context of future climate change. 相似文献
957.
Background
Evolution of parasite traits is inextricably linked to their hosts. For instance one common definition of parasite virulence is the reduction in host fitness due to infection. Thus, traits of infection must be viewed in both protagonists and may be under shared genetic and physiological control. We investigated these questions on the oomycete Hyaloperonospora arabidopsis (= parasitica), a natural pathogen of the Brassicaceae Arabidopsis thaliana. 相似文献958.
959.
Jill S. Marty James P. Higham Elizabeth L. Gadsby Caroline Ross 《International journal of primatology》2009,30(6):807-823
Sexual selection has driven the evolution and elaboration of a wide variety of displays and ornaments in male nonhuman primates,
including capes, cheek flanges, and sexual coloration. Among the most sexually dimorphic of all primates is the drill (Mandrillus leucophaeus), the males of which can be 3 times the mass of females, possess large canines, and exhibit extremely bright sexual skin
coloration. However, the function of male coloration in this species has never been examined. Here, we present data on male
color (measured objectively using digital photography), dominance rank, measures of male-female association, and key sexual
behaviors, of adult male drills (n = 17) living in four semi free-ranging enclosures at the Drill Rehabilitation and Breeding Center in Nigeria. We test the
hypothesis that male coloration is a badge of status, indicating dominance rank, and the hypothesis that male coloration attracts
females. We found that male coloration did indicate rank, and that high ranking, strongly colored males were more likely to
associate with adult females, and more specifically with fully tumescent females. These males also engaged in more sexual
activity. However, measures of male-female association and sexual behaviors were not related to male color once rank had been
taken into consideration; i.e., for males of a given rank, females did not prefer those that were more colorful. We discuss
the results in light of what is known about the wild drill social system, in which unfamiliar individuals may come together
in “supergroups,” and in which quality indicators of competitive ability may be particularly important. 相似文献