全文获取类型
收费全文 | 490篇 |
免费 | 4篇 |
国内免费 | 1篇 |
专业分类
系统科学 | 7篇 |
教育与普及 | 2篇 |
理论与方法论 | 11篇 |
现状及发展 | 35篇 |
研究方法 | 76篇 |
综合类 | 322篇 |
自然研究 | 42篇 |
出版年
2019年 | 2篇 |
2018年 | 5篇 |
2017年 | 12篇 |
2016年 | 6篇 |
2015年 | 6篇 |
2014年 | 3篇 |
2013年 | 3篇 |
2012年 | 48篇 |
2011年 | 80篇 |
2010年 | 13篇 |
2009年 | 1篇 |
2008年 | 53篇 |
2007年 | 42篇 |
2006年 | 36篇 |
2005年 | 40篇 |
2004年 | 34篇 |
2003年 | 40篇 |
2002年 | 43篇 |
2001年 | 3篇 |
2000年 | 3篇 |
1999年 | 5篇 |
1998年 | 2篇 |
1995年 | 1篇 |
1993年 | 1篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1975年 | 1篇 |
排序方式: 共有495条查询结果,搜索用时 736 毫秒
271.
272.
Kobasa D Jones SM Shinya K Kash JC Copps J Ebihara H Hatta Y Kim JH Halfmann P Hatta M Feldmann F Alimonti JB Fernando L Li Y Katze MG Feldmann H Kawaoka Y 《Nature》2007,445(7125):319-323
The 1918 influenza pandemic was unusually severe, resulting in about 50 million deaths worldwide. The 1918 virus is also highly pathogenic in mice, and studies have identified a multigenic origin of this virulent phenotype in mice. However, these initial characterizations of the 1918 virus did not address the question of its pathogenic potential in primates. Here we demonstrate that the 1918 virus caused a highly pathogenic respiratory infection in a cynomolgus macaque model that culminated in acute respiratory distress and a fatal outcome. Furthermore, infected animals mounted an immune response, characterized by dysregulation of the antiviral response, that was insufficient for protection, indicating that atypical host innate immune responses may contribute to lethality. The ability of influenza viruses to modulate host immune responses, such as that demonstrated for the avian H5N1 influenza viruses, may be a feature shared by the virulent influenza viruses. 相似文献
273.
Haag-Liautard C Dorris M Maside X Macaskill S Halligan DL Houle D Charlesworth B Keightley PD 《Nature》2007,445(7123):82-85
Spontaneous mutations are the source of genetic variation required for evolutionary change, and are therefore important for many aspects of evolutionary biology. For example, the divergence between taxa at neutrally evolving sites in the genome is proportional to the per nucleotide mutation rate, u (ref. 1), and this can be used to date speciation events by assuming a molecular clock. The overall rate of occurrence of deleterious mutations in the genome each generation (U) appears in theories of nucleotide divergence and polymorphism, the evolution of sex and recombination, and the evolutionary consequences of inbreeding. However, estimates of U based on changes in allozymes or DNA sequences and fitness traits are discordant. Here we directly estimate u in Drosophila melanogaster by scanning 20 million bases of DNA from three sets of mutation accumulation lines by using denaturing high-performance liquid chromatography. From 37 mutation events that we detected, we obtained a mean estimate for u of 8.4 x 10(-9) per generation. Moreover, we detected significant heterogeneity in u among the three mutation-accumulation-line genotypes. By multiplying u by an estimate of the fraction of mutations that are deleterious in natural populations of Drosophila, we estimate that U is 1.2 per diploid genome. This high rate suggests that selection against deleterious mutations may have a key role in explaining patterns of genetic variation in the genome, and help to maintain recombination and sexual reproduction. 相似文献
274.
A fruit of Eucommia has been recovered from the Eocene of Fushun, Liaoning Province. This represents the only confirmed occurrence of fruits of this genus in the paleobotanical record of China. Although Eucommia is endemic to central China today, the genus was more widespread in the Northern Hemisphere during the Tertiary. The fossil specimen from Fushun corresponds closely to the fruits of E. montana Brown from the Eocene of western North America and is easily distinguished from fruits of the living E. ulmoides Oliv. and the Japanese Eocene species, E. kobayashii Huzioka by its smaller size. 相似文献
275.
Sustainability is an important topic for understanding and developing our society (including business, government, and NGOs). For scholars who want their academic contributions to have an impact, sustainability is important for our conceptual systems (including theories, models, and policies). Because our conceptual systems share similarities with our social systems, we may investigate their characteristics to gain insight into how both may be achieved or at least understood. Theories of the humanities as well as the social/behavioral sciences are changing very rapidly. They are fragile and few seem to have any longevity. At the same time, the theoretical base does not seem to be “advancing.” They are not supporting highly effective results in the real world, so we continue to have seemingly insolvable problems such as crime, war, and poverty. This may be because academia has become inward-focused or, in Luhmann’s terminology, autonomous from the outside world. In seeking to understand how to develop more sustainable theories we found that the concept of sustainability is contested. And, in the process of comparing the sustainability of social systems to the sustainability of theories, we came to realize that neither perspective is viable. Drawing on Luhmann’s insights on the interdependence of theories and society, we came to realize that the two exist in a coevolutionary relationship. Importantly, we present an approach for measuring that evolution and suggest directions for accelerating the coevolutionary advance of society and science. 相似文献
276.
The following article has two parts. The first part recounts the history of a series of discoveries by Otto Neugebauer, Bartel van der Waerden, and Asger Aaboe which step by step uncovered the meaning of Column \(\varPhi \), the mysterious leading column in Babylonian System A lunar tables. Their research revealed that Column \(\varPhi \) gives the length in days of the 223-month Saros eclipse cycle and explained the remarkable algebraic relations connecting Column \(\varPhi \) to other columns of the lunar tables describing the duration of 1, 6, or 12 synodic months. Part two presents John Britton’s theory of the genesis of Column \(\varPhi \) and the System A lunar theory starting from a fundamental equation relating the columns discovered by Asger Aaboe. This article is intended to explain and, hopefully, to clarify Britton’s original articles which many readers found difficult to follow. 相似文献
277.
The importance of time‐varying volatility and country interactions in forecasting economic activity
下载免费PDF全文
![点击此处可从《Journal of forecasting》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Steven Trypsteen 《Journal of forecasting》2017,36(6):615-628
This paper examines the relative importance of allowing for time‐varying volatility and country interactions in a forecast model of economic activity. Allowing for these issues is done by augmenting autoregressive models of growth with cross‐country weighted averages of growth and the generalized autoregressive conditional heteroskedasticity framework. The forecasts are evaluated using statistical criteria through point and density forecasts, and an economic criterion based on forecasting recessions. The results show that, compared to an autoregressive model, both components improve forecast ability in terms of point and density forecasts, especially one‐period‐ahead forecasts, but that the forecast ability is not stable over time. The random walk model, however, still dominates in terms of forecasting recessions. 相似文献
278.
279.
Davies WI Zheng L Hughes S Tamai TK Turton M Halford S Foster RG Whitmore D Hankins MW 《Cellular and molecular life sciences : CMLS》2011,68(24):4115-4132
Melanopsin (OPN4) is an opsin photopigment that, in mammals, confers photosensitivity to retinal ganglion cells and regulates
circadian entrainment and pupil constriction. In non-mammalian species, two forms of opn4 exist, and are classified into mammalian-like (m) and non-mammalian-like (x) clades. However, far less is understood of the function of this photopigment family. Here we identify in zebrafish five
melanopsins (opn4m-1, opn4m-2, opn4m-3, opn4x-1 and opn4x-2), each encoding a full-length opsin G protein. All five genes are expressed in the adult retina in a largely non-overlapping
pattern, as revealed by RNA in situ hybridisation and immunocytochemistry, with at least one melanopsin form present in all
neuronal cell types, including cone photoreceptors. This raises the possibility that the teleost retina is globally light
sensitive. Electrophysiological and spectrophotometric studies demonstrate that all five zebrafish melanopsins encode a functional
photopigment with peak spectral sensitivities that range from 470 to 484 nm, with opn4m-1 and opn4m-3 displaying invertebrate-like
bistability, where the retinal chromophore interchanges between cis- and trans-isomers in a light-dependent manner and remains within the opsin binding pocket. In contrast, opn4m-2, opn4x-1 and opn4x-2
are monostable and function more like classical vertebrate-like photopigments, where the chromophore is converted from 11-cis to all-trans retinal upon absorption of a photon, hydrolysed and exits from the binding pocket of the opsin. It is thought that all melanopsins
exhibit an invertebrate-like bistability biochemistry. Our novel findings, however, reveal the presence of both invertebrate-like
and vertebrate-like forms of melanopsin in the teleost retina, and indicate that photopigment bistability is not a universal
property of the melanopsin family. The functional diversity of these teleost melanopsins, together with their widespread expression
pattern within the retina, suggests that melanopsins confer global photosensitivity to the teleost retina and might allow
for direct “fine-tuning” of retinal circuitry and physiology in the dynamic light environments found in aquatic habitats. 相似文献
280.
Fifteen widely separated sites within the pinyon - juniper woodlands of the Great Basin were cleared of trees. Understory response was recorded for 2 to 4 years. The array of postharvest plant assemblages were classified into one of four phytosociological groups using discriminant analysis. Pre - and postharvest plant assemblages from the same site appeared in the same phytosociological group, which indicates postharvest response could be predicted from the preharvest plant assemblage. Initial postharvest response appears cyclic in nature, and cycles are controlled by both residual plants and the rapid immigration of shrub species. Perennial plant density generally declined following tree harvest, but the fewer remaining plants produced significantly more cover than in preharvest stands. 相似文献