全文获取类型
收费全文 | 830468篇 |
免费 | 95047篇 |
国内免费 | 476篇 |
出版年
2018年 | 11256篇 |
2017年 | 10985篇 |
2016年 | 12056篇 |
2015年 | 13174篇 |
2014年 | 14947篇 |
2013年 | 20789篇 |
2012年 | 25912篇 |
2011年 | 30117篇 |
2010年 | 21451篇 |
2009年 | 20375篇 |
2008年 | 26800篇 |
2007年 | 28703篇 |
2006年 | 20954篇 |
2005年 | 21318篇 |
2004年 | 20568篇 |
2003年 | 20071篇 |
2002年 | 19421篇 |
2001年 | 34390篇 |
2000年 | 34866篇 |
1999年 | 27999篇 |
1998年 | 10725篇 |
1997年 | 11377篇 |
1996年 | 10924篇 |
1995年 | 10325篇 |
1994年 | 10176篇 |
1993年 | 10027篇 |
1992年 | 23591篇 |
1991年 | 22860篇 |
1990年 | 22479篇 |
1989年 | 21815篇 |
1988年 | 20077篇 |
1987年 | 19741篇 |
1986年 | 18108篇 |
1985年 | 18188篇 |
1984年 | 15125篇 |
1983年 | 13209篇 |
1982年 | 10556篇 |
1981年 | 9396篇 |
1980年 | 8949篇 |
1979年 | 14544篇 |
1978年 | 11690篇 |
1977年 | 10563篇 |
1976年 | 9961篇 |
1975年 | 10804篇 |
1974年 | 11626篇 |
1973年 | 11413篇 |
1972年 | 10640篇 |
1971年 | 9827篇 |
1970年 | 8113篇 |
1969年 | 7664篇 |
排序方式: 共有10000条查询结果,搜索用时 671 毫秒
731.
Jin Wei Mia Madel Alfajaro Peter C. DeWeirdt Ruth E. Hanna William J. Lu-Culligan Wesley L. Cai Madison S. Strine Shang-Min Zhang Vincent R. Graziano Cameron O. Schmitz Jennifer S. Chen Madeleine C. Mankowski Renata B. Filler Neal G. Ravindra Victor Gasque Fernando J. de Miguel Ajinkya Patil Huacui Chen Craig B. Wilen 《Cell》2021,184(1):76-91.e13
- Download : Download high-res image (212KB)
- Download : Download full-size image
732.
Charles L. Nunn Peter H. Thrall Kelly Stewart Alexander H. Harcourt 《Evolutionary ecology》2008,22(4):519-543
Emerging infectious diseases threaten a wide diversity of animals, and important questions remain concerning disease emergence
in socially structured populations. We developed a spatially explicit simulation model to investigate whether—and under what
conditions—disease-related mortality can impact rates of pathogen spread in populations of polygynous groups. Specifically,
we investigated whether pathogen-mediated dispersal (PMD) can occur when females disperse after the resident male dies from
disease, thus carrying infections to new groups. We also examined the effects of incubation period and virulence, host mortality
and rates of background dispersal, and we used the model to investigate the spread of the virus responsible for Ebola hemorrhagic
fever, which currently is devastating African ape populations. Output was analyzed using regression trees, which enable exploration
of hierarchical and non-linear relationships. Analyses revealed that the incidence of disease in single-male (polygynous)
groups was significantly greater for those groups containing an average of more than six females, while the total number of
infected hosts in the population was most sensitive to the number of females per group. Thus, as expected, PMD occurs in polygynous
groups and its effects increase as harem size (the number of females) increases. Simulation output further indicated that
population-level effects of Ebola are likely to differ among multi-male–multi-female chimpanzees and polygynous gorillas,
with larger overall numbers of chimpanzees infected, but more gorilla groups becoming infected due to increased dispersal
when the resident male dies. Collectively, our results highlight the importance of social system on the spread of disease
in wild mammals. 相似文献
733.
734.
735.
736.
737.
J. M. Warren 《Journal of human evolution》1974,3(6):445-454
The experimental literature on learning by Primate and non-Primate mammals is reviewed, with the aim of identifying peculiarly Primate features of learning. The evidence indicates that quantitative comparisons of learning by Primates and other mammals are intrinsically equivocal and uninformative because of the impossibility of equating experimental conditions for members of different species. The comparative results of early learning set studies were seriously misleading because the test conditions discriminated against representatives of species in which vision is not a dominant modality.Analyses of transfer between different learning tasks strongly suggest that rhesus macaques differ qualitatively from non-Primates like cats in that they develop generalized, trans-situationally valid response strategies during training on a particular problem which can facilitate learning in other situations. Non-Primate mammals appear not to develop such strategies under the same circumstances. There is also evidence that monkeys are more able than cats to discard previously learned strategies when they are no longer maximally profitable. 相似文献
738.
739.
740.
Genetically modified tobacco plants (Nicotiana tabacumSamsun)with antisense cinnamyl alcohol dehydrogenase DNA, produce secondaryxylem of a reduced tensile stiffness. These plants were grownalongside control plants. The stems of the plants were flexedor protected from flexing over a period of several weeks. Thetensile moduli and second moments of areas of the differenttissues inside the stems were measured and used to calculatethe bending stiffness of the plants. In tobacco, the cylinderof xylem was found to be the most important tissue in determiningthe bending stiffness of the plants. The thickness of the xylemtissue cylinder increased when plants were subjected to flexuralstimulation. This increased the bending stiffness of the stems.The response to mechanical stimulation was found to be correlatedwith tissue strain and the genetically modified plants wereable to exactly compensate for the reduced modulus of theirxylem tissue by increasing the thickness of the xylem tissuecylinder more than in control plants.Copyright 1999 Annals ofBotany Company. Tobacco plants, stem bending, xylem tissue, second moment of area, thigmomorphogenesis, mechanical strain. 相似文献