全文获取类型
收费全文 | 76119篇 |
免费 | 771篇 |
国内免费 | 1085篇 |
专业分类
系统科学 | 1631篇 |
丛书文集 | 834篇 |
教育与普及 | 606篇 |
理论与方法论 | 586篇 |
现状及发展 | 35060篇 |
研究方法 | 1309篇 |
综合类 | 35756篇 |
自然研究 | 2193篇 |
出版年
2014年 | 726篇 |
2013年 | 1107篇 |
2012年 | 1121篇 |
2011年 | 3266篇 |
2010年 | 866篇 |
2009年 | 1032篇 |
2008年 | 1482篇 |
2007年 | 1581篇 |
2006年 | 1517篇 |
2005年 | 1742篇 |
2004年 | 2715篇 |
2003年 | 2291篇 |
2002年 | 2024篇 |
2001年 | 2143篇 |
2000年 | 1634篇 |
1999年 | 1920篇 |
1998年 | 1311篇 |
1997年 | 1465篇 |
1996年 | 1189篇 |
1995年 | 950篇 |
1994年 | 1145篇 |
1993年 | 1140篇 |
1992年 | 1492篇 |
1991年 | 1274篇 |
1990年 | 1277篇 |
1989年 | 1169篇 |
1988年 | 1060篇 |
1987年 | 1046篇 |
1986年 | 963篇 |
1985年 | 1085篇 |
1984年 | 916篇 |
1983年 | 740篇 |
1982年 | 861篇 |
1981年 | 898篇 |
1980年 | 1020篇 |
1979年 | 1651篇 |
1978年 | 1481篇 |
1977年 | 1479篇 |
1976年 | 1263篇 |
1975年 | 1275篇 |
1974年 | 1289篇 |
1973年 | 1449篇 |
1972年 | 1539篇 |
1971年 | 1562篇 |
1970年 | 1763篇 |
1969年 | 1515篇 |
1968年 | 1479篇 |
1967年 | 1346篇 |
1966年 | 1128篇 |
1965年 | 873篇 |
排序方式: 共有10000条查询结果,搜索用时 25 毫秒
21.
Russell L. Burke Beverly Clendening Alexandra Kanonik 《Journal of Natural History》2018,52(25-26):1723-1732
Clutch size (CS) is a well-studied life history characteristic, and optimal egg size theory predicts that CS should correlate positively with reproductive investment. Turtles are good subjects for studies of reproductive strategies because few turtle species exhibit parental care; therefore quantifying their short-term reproductive investment is mostly limited to measuring egg size and number. Clutch size is usually reported as an average value for each turtle population, and where CS variation is noted, it is usually attributable to variation in adult body size. In two long-term studies of ecologically dissimilar species we detected a dramatic increase in CS in a common snapping turtle (Chelydra serpentina) population and in a diamondback terrapin (Malaclemys terrapin) population. It is unknown whether these changes are due to either variation in adult body size or resource availability; but the temporal patterns we observed have apparently never been reported previously. These trends remain unexplained and should be explored further. 相似文献
22.
23.
24.
Beryl?Royer-Bertrand Carlo?RivoltaEmail author 《Cellular and molecular life sciences : CMLS》2015,72(8):1463-1471
The past decade has seen the emergence of next-generation sequencing (NGS) technologies, which have revolutionized the field of human molecular genetics. With NGS, significant portions of the human genome can now be assessed by direct sequence analysis, highlighting normal and pathological variants of our DNA. Recent advances have also allowed the sequencing of complete genomes, by a method referred to as whole genome sequencing (WGS). In this work, we review the use of WGS in medical genetics, with specific emphasis on the benefits and the disadvantages of this technique for detecting genomic alterations leading to Mendelian human diseases and to cancer. 相似文献
25.
将风廓线雷达用于热带降水云体进行垂直探测,可以实时对热带地区降水云体进行较准确分类.通过对海口地区降水云系特征(2017年1月~12月)进行统计分析,提出一套基于风廓线雷达探测数据(回波强度、垂直速度和速度谱宽)的热带降水云体分类方法.以该方法为基础,结合天气雷达等观测资料,对2017年发生在海口地区的两次降水过程展开详细分析.结果表明:风廓线雷达具备较好体现降水云体垂直结构的能力,相比于天气雷达,风廓线雷达探测数据可以更精细地描述降水云系的变化趋势;相比于传统降水分类方案,采用以风廓线雷达探测数据为基础的分类方案可以提高对复杂降水类型识别的正确率;风廓线雷达可以更好地描述快速过境的高空微弱对流系统. 相似文献
26.
To evaluate the biocide effect of quaternary ammonium chloride (QAC), a whitewater sample was taken from a fine papermachine headbox. By plate spreading method, 51 strains of facultative anaerobe were isolated morphologically. Then the strains were separately transferred to basal medium and were incubated before the beginning of log phase. To identify strains with different QAC resistance, 30-120 ppm of N-Alkyl-benzyl-dimethyl ammonium chloride were added to basal medium.Biocide effect was investigated by comparison of bacterial growth, which can be monitored by 600 nm light absorbance of basal medium suspension. Among 51 strains, only 2 strains can Survive for QAC concentration up to 120 ppm. API20E aud 16S rRNA gene sequencing technique were applied to identify two strains with highest (120 ppm)QAC resistance. One strain (HB22)was identified as Morganella morganii. HB22 can resist QAC concentration up to 150 ppm. HB22 is Gram-negative rod, motile with flagella, catalase positive, oxidase negative, Indole positive, H2S production negative, facultative anaerobe. HB22 has optimum growth condition of 35℃ and pH 7.0.HB22 can catabolize only glucose and D (+)-Mannose.The other (HB45) was identified with high similarity among Pseudomonas cf. monteili or Pseudomonas mosselii or Pseudomonas putida. HB45 can resist QAC concentration up to 200 ppm.HB45 is Gram-negative rod, motile with flagella,catalase positive, oxidase negative, Indole negative,H2S production negative, aerobe. HB45 has optimum growth condition of 30℃ and pH 7.4. HB45 can catabolize L-Arabinose, L(+)-Rhanmose, D (+)-Mannose, Glucose, Glycerol, Lactose, Maltose,Raffmose, Xylose, Cellulose and Xylan. The implication of this work to paper industry is also discussed. 相似文献
27.
28.
29.
Mark H. Bickhard 《Foundations of Science》2003,8(3):283-293
If the general arguments concerning theinvolvement of variation and selection inexplanations of ``fit'' are valid, then variationand selection explanations should beappropriate, or at least potentiallyappropriate, outside the paradigm historisticdomains of biology and knowledge. In thisdiscussion, I wish to indicate some potentialroles for variation and selection infoundational physics – specifically inquantum field theory. I will not be attemptingany full coherent ontology for quantum fieldtheory – none currently exists, and none islikely for at least the short term future. Instead, I wish to engage in some partiallyspeculative interpretations of some interestingresults in this area with the aim ofdemonstrating that variation and selectionnotions might play a role even here. Ifvariation and selection can survive in even asinhospitable and non-paradigmatic a terrain asfoundational physics, then it can surviveanywhere. 相似文献
30.
自主机器人人工进化方法研究 总被引:1,自引:0,他引:1
在介绍人工进化研究状况的基础上 ,着重阐述了进化机器人学的关键问题 :基本概念、进化技术、进化计算和实验研究方法等 ,探索了进化机器人学未来的研究趋势及所面临的挑战 . 相似文献