全文获取类型
收费全文 | 62587篇 |
免费 | 5187篇 |
国内免费 | 31篇 |
专业分类
67805篇 |
出版年
2022年 | 442篇 |
2021年 | 914篇 |
2020年 | 556篇 |
2019年 | 678篇 |
2018年 | 878篇 |
2017年 | 811篇 |
2016年 | 1433篇 |
2015年 | 2542篇 |
2014年 | 2656篇 |
2013年 | 3470篇 |
2012年 | 4450篇 |
2011年 | 4415篇 |
2010年 | 2863篇 |
2009年 | 2537篇 |
2008年 | 3659篇 |
2007年 | 3735篇 |
2006年 | 3512篇 |
2005年 | 3541篇 |
2004年 | 3454篇 |
2003年 | 3242篇 |
2002年 | 3223篇 |
2001年 | 732篇 |
2000年 | 545篇 |
1999年 | 748篇 |
1998年 | 888篇 |
1997年 | 610篇 |
1996年 | 624篇 |
1995年 | 602篇 |
1994年 | 587篇 |
1993年 | 605篇 |
1992年 | 544篇 |
1991年 | 470篇 |
1990年 | 399篇 |
1989年 | 410篇 |
1988年 | 411篇 |
1987年 | 348篇 |
1986年 | 348篇 |
1985年 | 408篇 |
1984年 | 461篇 |
1983年 | 386篇 |
1982年 | 466篇 |
1981年 | 417篇 |
1980年 | 365篇 |
1979年 | 248篇 |
1978年 | 298篇 |
1977年 | 288篇 |
1976年 | 233篇 |
1975年 | 220篇 |
1974年 | 240篇 |
1973年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
51.
During their 120 to 165 million years of isolation, the flora and fauna of Madagascar evolved, to a large extent, independently of the African mainland.1 In contrast to other oceanic islands, Madagascar is large enough to house the major components of tropical ecosystems, allowing tests of evolutionary hypotheses on the level of complete communities. Taking lemurs, the primates of Madagascar, as an example, evolutionary hypotheses correctly predict the organization of their community structure with respect to ecological correlates. Lemur social systems and their morphological correlates, on the other hand, deviate in some respects from those of other primates. Apparently, lemur social systems are influenced by several selection pressures that are weak or rare in other primates. These include variable activity patterns and avoidance of infanticide. The interspecific variation in lemur social systems therefore offers a unique opportunity for a comprehensive study of the determinants of primate social systems. 相似文献
52.
A lightweight finger printing stand is described which can be adjusted to the proper printing height. Based upon experience printing over 1,100 subjects, 12 advantages of using the stand are suggested. 相似文献
53.
Ralf J. Jäger Vincent R. Harley Rudolf A. Pfeiffer Peter N. Goodfellow Gerd Scherer 《Human genetics》1992,90(4):350-355
A familial mutation in SRY, the gene coding for the testis-determining factor TDF, was identified in an XY female with gonadal dysgenesis, her father, her two brothers and her uncle. The mutation consists of a T to C transition in the region of the SRY gene coding for a protein motif known as the high mobility group (HMG) box, a protein domain known to confer DNA-binding specificity on the SRY protein. This point mutation results in the substitution, at amino acid position 109, of a serine residue for phenylalanine, a conserved aromatic residue in almost all HMG box motifs known. This F109S mutation was not found in 176 male controls. When recombinant wildtype SRY and SRYF109S mutant protein were tested in vitro for binding to the target site AAC AAAG, no differences in DNA-binding activity were observed. These results imply that the F109S mutation either is a rare neutral sequence variant, or produces an SRY protein with slightly altered in vivo activity, the resulting sex phenotype depending on the genetic back-ground or environmental factors.This paper is dedicated by G. S. to Professor Ulrich Wolf on the occasion of his 60th birthday 相似文献
54.
55.
56.
57.
58.
59.
60.
Aquatic environmental impact associated with stream-crossing by a pipeline was monitored at Archibald Creek, B.C. for two years. Water chemistry and benthic macroinvertebrates were used as monitoring tools. Results indicated that impacts arising from stream-crossing were short-term and non-residual.Funded by a contract from Canadian Arctic Gas Study Limited, calgary, to Aquatic Environments Limited, Calgary.Aquatic Environments Limited, Calgary.Aquatic Environments Limited, Calgary. 相似文献