首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   363934篇
  免费   40452篇
  国内免费   265篇
  2018年   3501篇
  2017年   3339篇
  2016年   4837篇
  2015年   6721篇
  2014年   7767篇
  2013年   10950篇
  2012年   12410篇
  2011年   12416篇
  2010年   8216篇
  2009年   7157篇
  2008年   10767篇
  2007年   11164篇
  2006年   10548篇
  2005年   10143篇
  2004年   10145篇
  2003年   9759篇
  2002年   9656篇
  2001年   15450篇
  2000年   15523篇
  1999年   12449篇
  1998年   4428篇
  1997年   4471篇
  1996年   4255篇
  1995年   4095篇
  1994年   4119篇
  1993年   3987篇
  1992年   10422篇
  1991年   9976篇
  1990年   9781篇
  1989年   9493篇
  1988年   8856篇
  1987年   8451篇
  1986年   7665篇
  1985年   7763篇
  1984年   6433篇
  1983年   5652篇
  1982年   4491篇
  1981年   4022篇
  1980年   3750篇
  1979年   6506篇
  1978年   4865篇
  1977年   4546篇
  1976年   4203篇
  1975年   4612篇
  1974年   4953篇
  1973年   4995篇
  1972年   4657篇
  1971年   4339篇
  1970年   3646篇
  1969年   3556篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
113.
Sexually selected infanticide is an important source of infant mortality in many mammalian species. In species with long-term male-female associations, females may benefit from male protection against infanticidal outsiders. We tested whether mountain gorilla (Gorilla beringei beringei) mothers in single and multi-male groups monitored by the Dian Fossey Gorilla Fund’s Karisoke Research Center actively facilitated interactions between their infants and a potentially protective male. We also evaluated the criteria mothers in multi-male groups used to choose a preferred male social partner. In single male groups, where infanticide risk and paternity certainty are high, females with infants <1 year old spent more time near and affiliated more with males than females without young infants. In multi-male groups, where infanticide rates and paternity certainty are lower, mothers with new infants exhibited few behavioral changes toward males. The sole notable change was that females with young infants proportionally increased their time near males they previously spent little time near when compared to males they had previously preferred, perhaps to encourage paternity uncertainty and deter aggression. Rank was a much better predictor of females’ social partner choice than paternity. Older infants (2–3 years) in multi-male groups mirrored their mothers’ preferences for individual male social partners; 89% spent the most time in close proximity to the male their mother had spent the most time near when they were <1 year old. Observed discrepancies between female behavior in single and multi-male groups likely reflect different levels of postpartum intersexual conflict; in groups where paternity certainty and infanticide risk are both high, male-female interests align and females behave accordingly. This highlights the importance of considering individual and group-level variation when evaluating intersexual conflict across the reproductive cycle.  相似文献   
114.
115.
Nonpigmented and late-pigmenting rapidly growing mycobacteria (RGM) have been reported to commonly colonize water production and distribution systems. However, there is little information about the nature and distribution of RGM species within the different parts of such complex networks or about their clustering into specific RGM species communities. We conducted a large-scale survey between 2007 and 2009 in the Parisian urban tap water production and distribution system. We analyzed 1,418 water samples from 36 sites, covering all production units, water storage tanks, and distribution units; RGM isolates were identified by using rpoB gene sequencing. We detected 18 RGM species and putative new species, with most isolates being Mycobacterium chelonae and Mycobacterium llatzerense. Using hierarchical clustering and principal-component analysis, we found that RGM were organized into various communities correlating with water origin (groundwater or surface water) and location within the distribution network. Water treatment plants were more specifically associated with species of the Mycobacterium septicum group. On average, M. chelonae dominated network sites fed by surface water, and M. llatzerense dominated those fed by groundwater. Overall, the M. chelonae prevalence index increased along the distribution network and was associated with a correlative decrease in the prevalence index of M. llatzerense, suggesting competitive or niche exclusion between these two dominant species. Our data describe the great diversity and complexity of RGM species living in the interconnected environments that constitute the water production and distribution system of a large city and highlight the prevalence index of the potentially pathogenic species M. chelonae in the distribution network.  相似文献   
116.
117.
118.
Six hundred foramina transversaria (FT) of the axis vertebrae of 4 population groups were examined for the presence of erosion of the lateral articular mass. Of these, 127 (21%) and 72 (12%) presented with moderate and marked erosion, respectively. Evidence is presented incriminating the tortuosity of the vertebral artery as well as congenital factors in the etiology of the erosion of the FT. Attention is drawn to the possible correlation between angulation of the transverse process of the axis and erosion of the FT.  相似文献   
119.
Two internal herinias of the intestines were found in adult males. One was a large translucent avascular membranous sac contining the small intestine from the duodenojejunal flexure to a point 6 in. proximal to the ileocaecal junction. The other was a peritoneal sac enclosing the small intestine, appendix, caecum and 6 in. of the ascending colon. The mesenteric and colic vessels were normal. Both hernias conformed to PAPEZ's concept of the so-called paraduodenal hernia that the hernial sac is derived from the umbilical coelom. The authors suggest that most of the so-called paraduodenal hernias are derived from the embryonic umbilical peritoneal diverticulum rather than from the peritoneal recesses or mesentery of the colon.  相似文献   
120.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号