全文获取类型
收费全文 | 4792篇 |
免费 | 161篇 |
出版年
2020年 | 48篇 |
2019年 | 103篇 |
2018年 | 117篇 |
2017年 | 118篇 |
2016年 | 138篇 |
2015年 | 104篇 |
2014年 | 93篇 |
2013年 | 1411篇 |
2012年 | 144篇 |
2011年 | 125篇 |
2010年 | 155篇 |
2009年 | 155篇 |
2008年 | 109篇 |
2007年 | 126篇 |
2006年 | 107篇 |
2005年 | 71篇 |
2004年 | 59篇 |
2003年 | 60篇 |
2002年 | 64篇 |
2001年 | 74篇 |
2000年 | 58篇 |
1999年 | 72篇 |
1998年 | 66篇 |
1997年 | 58篇 |
1996年 | 59篇 |
1995年 | 46篇 |
1994年 | 56篇 |
1993年 | 51篇 |
1992年 | 48篇 |
1991年 | 39篇 |
1990年 | 27篇 |
1989年 | 45篇 |
1988年 | 38篇 |
1987年 | 51篇 |
1986年 | 41篇 |
1985年 | 46篇 |
1984年 | 55篇 |
1983年 | 52篇 |
1982年 | 34篇 |
1981年 | 46篇 |
1980年 | 59篇 |
1979年 | 48篇 |
1978年 | 30篇 |
1977年 | 54篇 |
1976年 | 34篇 |
1975年 | 39篇 |
1974年 | 23篇 |
1972年 | 26篇 |
1970年 | 23篇 |
1969年 | 23篇 |
排序方式: 共有4953条查询结果,搜索用时 15 毫秒
1.
Evidence from several disciplines supports the proposition that following the Wisconsinan and an initial but brief moist phase of the Holocene, there was a gradual drying trend in the mid-continent Prairie Peninsula that peaked about 7000 years ago, a pattern that was not reversed until about 4000 years ago. Except for minor perturbations, conditions during the past four millennia have essentially been similar to those of the present. Studies in palynology and geomorphology have shown that the climatic regimes of the early/mid Post-glacial had a marked influence on both the biota and landscape erosional patterns, especially along the margins of the Prairie Peninsula. Measurable changes in the landscape and attendant biota show a time-transgressive pattern of drought related phenomena recorded palaeo-ecologically from west to east.Few studies of archaeological sites in the Prairie Peninsula have examined the evidence in and around the sites themselves for data reflecting local environmental dynamics. In fact, earlier models proposed for explaining the evolution of food-procuring systems in eastern North America virtually ignored potential changes in the natural environment. This paper examines data from two archaeological sites along the prairie-forest border in Missouri that contain long cultural sequences. Evidence is presented that pronounced changes in the natural environment occurred and that these biophysical variables are indeed critical for understanding the evolution of food-procuring societies in the American Midwest. 相似文献
2.
3.
4.
5.
6.
7.
8.
Susan R. Brooker-Gross 《Journal of Historical Geography》1981,7(2):167-179
News wire services were established in the United States in the mid-nineteenth century to gather and distribute news for local newspapers, using the new technology of the telegraph. With that early beginning in the era of electronic communication, the expansion of news wire services spans the change from a society of local communities to a more nationally integrated society. This paper assesses the importanceof changes in the medium's spatial organization which promoted national culture. The first wire services were local, ad hoc groups, organized to achieve greater efficiency in news-gathering. Formal services were first organized on a local basis, but in 1848 they began to sell news to the major urbanized portions of the United States. As the nation grew, the desire for greater efficiency conflicted with the desire for greater autonomy. Regional wire services persisted from the 1860s to the 1880s but after the 1880s, a tenuous national organization was achieved, cementing itself into a national system after the turn of the century. Efficiency in news collection was the primary reason for the increasing scale of news collecting groups. Each step toward larger spatial coverage, fed the growth of a complex interdependent urban system. 相似文献
9.
10.