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   494篇
  免费   5篇
  2018年   1篇
  2017年   1篇
  2016年   2篇
  2015年   2篇
  2014年   4篇
  2013年   21篇
  2012年   19篇
  2011年   25篇
  2010年   35篇
  2009年   35篇
  2008年   35篇
  2007年   41篇
  2006年   24篇
  2005年   20篇
  2004年   10篇
  2003年   8篇
  2002年   5篇
  2001年   6篇
  2000年   6篇
  1999年   6篇
  1998年   11篇
  1997年   14篇
  1996年   17篇
  1995年   12篇
  1994年   14篇
  1993年   11篇
  1992年   11篇
  1991年   12篇
  1990年   11篇
  1989年   10篇
  1988年   8篇
  1987年   5篇
  1986年   11篇
  1985年   8篇
  1984年   6篇
  1983年   2篇
  1982年   4篇
  1981年   4篇
  1980年   2篇
  1979年   3篇
  1978年   5篇
  1976年   2篇
  1975年   4篇
  1973年   2篇
  1971年   1篇
  1967年   1篇
  1962年   1篇
  1954年   1篇
排序方式: 共有499条查询结果,搜索用时 62 毫秒
51.
In this article, I consider the effects of Spanish missionization on indigenous identity and biological interaction patterns. Odontometric data were recorded for 26 skeletal samples dating to three time periods: precontact (C.E. 1200–1400), early mission (C.E. 1600–1650), and late mission (C.E. 1650–1700). Population genetic analyses generated estimates of regional genetic variation (FST) and intersample genetic distances. Genetic variation during the precontact period was limited despite documented linguistic and cultural variation. Variation increased during the early mission period, indicating a decline in between-group interaction despite inclusion within a single colonial sociopolitical framework. During the late mission period, variation declined significantly, indicating a dramatic reduction in between-group variability consistent with genetic drift and gene flow between communities. I discuss these results in terms of archaeological and historical models of postcolonial transformation and suggest an emerging polyethnic community was resident in La Florida preceding the subsequent diaspora caused by burgeoning European military conflict in eastern North America.  相似文献   
52.
53.
Time series data from the Defense Meteorological Satellite Program (DMSP) Operational Linescan System (OLS) have been used to derive georeferenced inventories of human settlements for Europe, North and South America, and Asia. The visible band of the OLS is intensified at night, permitting detection of nocturnal visible-near infrared emissions from cities, towns, and villages. The time series analysis makes it possible to eliminate ephemeral VNIR emission sources such as fire and to normalize for differences in the number of cloud-free observations. An examination of the area lit (km2) for 52 countries indicates the OLS derived products may be used to perform the spatial apportionment of population and energy related greenhouse gas emissions.  相似文献   
54.
Cultured eggs of Cyathocephalus truncatus were found to develop optimally at 20°C, but they did not develop at 6°C and above. The oncosphere of the tapeworm has been seen to be a true hexacanth embryo. Specimens of the intermediate host. Gammaurs pulex , were infected by being forced to feed on pieces of lettuce leaves on which infective eggs of the tapeworm were placed. The young drveloping porocercoid was first recovered from the haemocoel of Gammarus pulex 5 days after infection. The procercoid was found to reach the infective stage within the amphipod after about ten weeks.  相似文献   
55.
56.
A compromise between speed and efficiency of energy conversionsgives the maximum power of useful energy conversion at intermediateefficiencies. Organisms are selected to maximize the power ofenergy conversions to a useful form. However, most species havevery little capacity to vary the efficiency of their energyconversions in response to variation in the intensity of theenvironmental power supply. Plants can respond slowly to horizontalvariation in the availability of energy by growth. The trade-offbetween power and efficiency which is dependent on the compromisebetween speed and efficiency of energy conversions does seemto apply to the relative efficiencies of successive speciesin a sere of secondary terrestrial plant succession. The applicationof the power trade-off to species in a sere predicts the commongeneral properties of growth and reproduction in succession.The power trade-off may also help to explain reproductive patternsin animals that differ in the concentration of their food supply.The thermodynamic basis of causation provided by the power trade-offcould be a valuable tool for connecting evol utionary ecologywith community and ecosystem studies.  相似文献   
57.
58.
Abstract. 1. The population of the lycosid Trochosa terricola Thorell was sampled from April 1973 to August 1975 at Weeting Heath NNR, a Breckland grass heath.
2. Four sampling methods were compared for efficiency. Hand searching gave density estimates between 38.3 and 70.1% of heat extraction.
3. The temperature range in the sward at +1 cm was –5°C to 39°C with January and July means of 3.2°C and 17.4°C.
4. Eight male and nine female instars were determined and the life cycle extended over 2 or 3 years.
5. Adults were nocturnal but the juveniles diurnal. An annual diplochrone activity pattern was observed for adult males.
6. The horizontal distribution within the sward was aggregated, the structure and microhabitat being important determining factors. The population density was greater in moist, young Festuca spp. tussocks. An equation relating population density to habitat characteristics was derived.
7. The overall population density ranged from 14.0 m-2 to 76.0 m-2 and was maximal in autumn after breeding. The population biomass was greatest during autumn (291.2 mg d.wt m-2).
8. Mean number of juveniles emerging from an egg sac was 77.3 (first sac) and 38.0 (second sac). The natality in 1973 was 66.8 individuals m-2.
9. The survivorship curve until maturity varied between types I and III in different years.
10. The population dynamics were compared and are discussed in the light of other data. The variable population characteristics suggested that Den Boer's "spreading of risk" theory applied to the T.terricola population.  相似文献   
59.
Abstract. To study the impact of residual pyrethroid insecticide on the abundance and distribution of peridomestic Lutzomyia longipalpis , the sandfly vector of visceral leishmaniasis in Brazil, lambda-cyhalothrin was applied at 20 mg a.i.m-2 in the following interventions: (i) spraying of all animal pens in a village (blanket coverage); (ii) treatment of a subset of animal pens, either by spraying, or by installation of insecticide-impregnated 1 m2 cotton sheets as 'targets' (focal coverage).
By sampling with CDC light traps, and using a novel analytical approach, we detected a 90% reduction in Lu. longipalpis abundance in sprayed sheds of the focal intervention. However, there was no discernible effect on the abundance of other phlebotomines trapped in sheds, or on the abundance of Lu. longipalpis in untreated dining-huts and houses. This differential impact on Ludongipalpis abundance is explained in terms of the disruption of male pheromone production. Treated targets were approximately half as effective as residual spraying in reducing the abundance of Lu.longipalpis in sheds.
Following blanket intervention, the abundance of Lu.longipalpis in traps fell by only 45% (not significant): catches at untreated dining-huts actually increased, possibly because the blanket coverage diverted Lu. longipalpis away from major aggregation sites at animal pens. It is recommended that care be taken during vector control programmes to ensure that all potential aggregation sites are treated. The possible consequences of leaving some sites untreated include poor control of peridomestic sandfly abundance and an increase in the biting rate on dogs and humans.  相似文献   
60.
Species serve as both the basic units of macroevolutionary studies and as the basic units of taxonomic classification. In this paper I argue that the taxa identified as species by the Phylogenetic Species Concept (Mishler and Brandon 1987) are the units of biological organization most causally relevant to the evolutionary process but that such units exist at multiple levels within the hierarchy of any phylogenetic lineage. The PSC gives us no way of identifying one of these levels as the privileged level on which taxonomic classifications can be based.  相似文献   
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