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   701篇
  免费   59篇
  2024年   2篇
  2023年   23篇
  2022年   4篇
  2021年   9篇
  2020年   35篇
  2019年   35篇
  2018年   104篇
  2017年   73篇
  2016年   66篇
  2015年   48篇
  2014年   42篇
  2013年   153篇
  2012年   34篇
  2011年   24篇
  2010年   14篇
  2009年   14篇
  2008年   16篇
  2007年   12篇
  2006年   4篇
  2005年   2篇
  2004年   2篇
  2003年   1篇
  2002年   4篇
  2001年   9篇
  2000年   3篇
  1999年   2篇
  1998年   1篇
  1997年   3篇
  1996年   2篇
  1995年   7篇
  1994年   2篇
  1993年   2篇
  1992年   3篇
  1989年   2篇
  1982年   1篇
  1980年   1篇
  1979年   1篇
排序方式: 共有760条查询结果,搜索用时 15 毫秒
131.
Abstract

The central focus of this work is to test a new methodology to measure the impact of the railway on the distribution of population, in this case in Spain. To achieve this, it was necessary to previously integrate data relating to population and railway lines into a geographical information system. The result was a spatial database that includes population data from homogeneous census series obtained for the municipal scale and the evolution of the railway network in service at corresponding points in time. This allowed the authors to apply spatial-temporal analysis. By so doing, this work constitutes an analysis of a new methodology, as they used exploratory spatial data analysis and geographically weighted regression to detect spatial patterns and estimate the influence of the railway and distance from the coast on population change. The results obtained show that the influence of the railway was very pronounced in some areas, while in others it was just one of the factors that could explain major changes in population distribution.  相似文献   
132.
Abstract

The introduction of the railway network brought with it an unprecedented improvement in accessibility. In this work, the authors analyze the evolution of the territorial coverage of the railway network and its influence on the uneven distribution of population. To carry out this research, they used comparable data on total population obtained from census records relating to civil parishes of England and Wales, taken at 10-year intervals from 1871. The hypothesis that they wished to test was that good access to a railway station was related to significant increases in population. This exercise provides a better understanding of regional variations in population growth and allows the authors to identify current differences between urban and rural areas that have resulted from their historical evolution.  相似文献   
133.
134.
135.
136.
137.
138.
Domestic Pigs in Africa   总被引:1,自引:0,他引:1  
The history of African pig breeds is still controversial due to the relative scarcity of archaeological and genetic data clarifying their origins. While these breeds might descend from Near Eastern pigs entering the continent via Egypt during the Neolithic, Africa may be a primary domestication locus for pigs, as indicated for other species such as cattle and donkeys. In this regard, characterisation of the mitochondrial gene pool of African pig breeds has revealed a very low frequency of Near Eastern alleles, suggesting that, if Fertile Crescent pigs played a part in the foundation of African breeds, their genetic signature has been substantially erased. Interestingly, genetic analysis of western and eastern African pig breeds has revealed a strong phylogeographic pattern, with the latter harbouring Far Eastern alleles at high frequencies. This finding is consistent with data obtained for chickens and confirms that livestock was transported in ancient times, from the Far East to Africa as a consequence of the Indian Ocean trade. European colonisation of Africa also involved the introduction of exotic swine breeds such as Iberian pigs. The confluence of the highly divergent European and Far Eastern Sus scrofa gene pools contributed to significantly enrich the genetic reservoir of African swine breeds, favouring their adaptation to environmental conditions that are often harsh. Conservation of this genetic legacy will be of utmost importance to ensure the prosperity of current resource-based subsistence farming systems in Africa.  相似文献   
139.
The objective of this paper is to assess the relationship between the cereals cultivated in the Late Bronze and Early Iron Ages (ca. 1250–400 BC) within the area of the present-day Czech Republic, and their environmental settings. The various charred caryopses of cereal species represented in the archaeobotanical assemblages from 35 archaeological sites differ, especially in the proportion of wheat and barley. The cereal assemblages were compared regarding site altitude, weather conditions, soils and soil productivity. The most important environmental variable influencing the choice of a particular crop seemed to be altitude which is correlated with other variables such as the length of growing season, mean annual temperature, soil quality etc. Although the ecological requirements of cereals cultivated in the Late Bronze and Early Iron Ages are not known, they presumably thrived under similar conditions to present-day species/varieties, and the strategy of past crop husbandry was based on similar principles as today, e.g. flexible adaptation to local environmental conditions, in an effort to achieve optimal yields and reduce the danger of crop failure.  相似文献   
140.
Over one hundred years of palaeontological research in northwestern Argentina has provided extensive knowledge of Andean lower Palaeozoic fossil assemblages, with trilobites, graptolites, brachiopods and echinoderms being among the most prominent groups. This record is enriched by the recent discovery of soft-bodied worms in Cambrian outcrops of northwestern Argentina. Palaeoscolex sp. cf. P. ratcliffei from the Furongian Lampazar Formation in Jujuy is described, considerably expanding the biogeographical range of this genus and filling the distributional gap, between the well-known early–middle Cambrian occurrences of Palaeoscolex and those of its Ordovician species.  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