全文获取类型
收费全文 | 4273篇 |
免费 | 172篇 |
国内免费 | 277篇 |
专业分类
4722篇 |
出版年
2023年 | 26篇 |
2022年 | 53篇 |
2021年 | 46篇 |
2020年 | 51篇 |
2019年 | 73篇 |
2018年 | 82篇 |
2017年 | 105篇 |
2016年 | 108篇 |
2015年 | 90篇 |
2014年 | 135篇 |
2013年 | 216篇 |
2012年 | 87篇 |
2011年 | 151篇 |
2010年 | 127篇 |
2009年 | 244篇 |
2008年 | 299篇 |
2007年 | 256篇 |
2006年 | 236篇 |
2005年 | 210篇 |
2004年 | 157篇 |
2003年 | 138篇 |
2002年 | 104篇 |
2001年 | 104篇 |
2000年 | 98篇 |
1999年 | 113篇 |
1998年 | 81篇 |
1997年 | 93篇 |
1996年 | 101篇 |
1995年 | 82篇 |
1994年 | 83篇 |
1993年 | 77篇 |
1992年 | 87篇 |
1991年 | 60篇 |
1990年 | 67篇 |
1989年 | 71篇 |
1988年 | 63篇 |
1987年 | 81篇 |
1986年 | 48篇 |
1985年 | 52篇 |
1984年 | 36篇 |
1983年 | 14篇 |
1982年 | 27篇 |
1981年 | 32篇 |
1980年 | 27篇 |
1979年 | 27篇 |
1978年 | 23篇 |
1977年 | 19篇 |
1976年 | 23篇 |
1975年 | 15篇 |
1973年 | 22篇 |
排序方式: 共有4722条查询结果,搜索用时 0 毫秒
211.
Colin J. Bibby 《Bird Study》2013,60(3):194-210
Capsule Field ornithology is alive and well, and in the future can contribute much more in Britain and elsewhere. Aims To review the progress of field ornithology in Britain in the context of Bird Study and the British Trust for Ornithology. Methods An overview was taken of the main subject areas published in Bird Study to characterize progress over 50 years. Some quantification of data on the main bird organizations was performed. ResultsKnowledge of status and trends of British birds has moved from the qualitative to generally well detailed quantitative over the past 50 years. Causes of distribution and changes are increasingly well understood in terms of habitat and of population processes. Behaviour and ecology have grown to be separate disciplines in their own right although birds have been major subjects of study within them. Migration studies have lagged somewhat in Britain but advanced elsewhere in recent decades. Amongst the main bird organizations, the BTO still has a growing membership, that of the RSPB has reached an astonishing level but may be nearing a plateau while the BOU's membership is clearly in decline. Conclusions Field ornithology has made a large contribution to the environmental debate and the future is set for further integration across disciplines in answering large-scale questions. Great public interest has supported the growth of ornithology with a big switch from amateur to professional workers. There is a major challenge to exploit the value of birds in promoting interest in the environment and delivering sound facts to support the biodiversity debate elsewhere on Earth. Fifty years of history in Britain indicate what is possible. 相似文献
212.
Considering the addition of intermediate transmission steps during life cycle evolution, developmental plasticity, canalization forces and inherited parental effect must be invoked to explain new host colonization. Unfortunately, there is a lack of experimental procedures and relevant models to explore the adaptive value of alternative developmental phenotypes during life cycle evolution. However, within the monogeneans that are characterized by a direct life cycle, an extension of the transmission strategy of amphibian parasites has been reported within species of Polystoma and Metapolystoma (Polyopisthocotylea; Polystomatidae). In this study, we tested whether the infection success of Polystoma gallieni within tadpoles of its specific host, the Stripeless Tree Frog Hyla meridionalis, differs depending on the parental origin of the oncomiracidium. An increase in the infection success of the parasitic larvae when exposed to the same experimental conditions as their parents was expected as an adaptive pattern of non-genetic inherited information. Twice as many parasites were actually recorded from tadpoles infected with oncomiracidia hatching from eggs of the bladder parental phenotype (1.63 ± 0.82 parasites per host) than from tadpoles infected with oncomiracidia hatching from eggs of the branchial parental phenotype (0.83 ± 0.64 parasites per host). Because in natural environments the alternation of the two phenotypes is likely to occur due to the ecology of its host, the differential infection success within young tadpoles could have an adaptive value that favors the parasite transmission over time. 相似文献
213.
214.
Yaoyu Feng Eucaris Torres Na Li Lin Wang Dwight Bowman Lihua Xiao 《International journal for parasitology》2013
The subtype IIaA15G2R1 at the 60 kDa glycoprotein (gp60) gene locus is the most dominant Cryptosporidium parvum infecting dairy cattle and humans in industrialised nations. The reasons for its high transmissibility are not clear, and it remains to be determined whether this subtype represents a homogeneous parasite population. In this study, we sequence-characterised 26 IIaA15G2R subtype specimens and 26 non-IIaA15G2R subtype specimens from the United States, Canada, United Kingdom and Spain at seven other known polymorphic loci, including CP47, CP56, DZ-HRGP, MSC6-5, MSC6-7, RPGR and ZPT. Extensive heterogeneity within IIaA15G2R1 and discordance in typing results between gp60 and other genetic markers were observed. Results of inter-locus and intra-ZPT linkage disequilibrium and recombination analyses indicated that the heterogeneity within IIaA15G2R1 and discordance in typing results among genetic loci were largely due to the occurrence of genetic recombination, mostly within the gp60 subtype IIaA15G2R1. Although there was no clear population diversion between IIaA15G2R and non-IIaA15G2R subtypes, results of STRUCTURE and FST analyses suggested the presence of at least two subpopulations; subpopulation 1 had an epidemic population structure and was widely distributed, whereas subpopulation 2 had a clonal population structure and consisted of geographically segregated multilocus subtypes. Genetic recombination between epidemic and geographically segregated C. parvum populations appeared to be a driving force in the emergence of a hyper-transmissible IIaA15G2R1 subtype. Genetic recombination was observed even between the zoonotic IIa subtype family and anthroponotic subtype family IIc at CP56, MSC6-7 and ZPT. Thus, the IIaA15G2R1 subtype at gp60 is likely a fitness marker for C. parvum and the wide spread of IIaA15G2R1 subtype around the world is probably independent of the sequence characteristics at other genetic loci. 相似文献
215.
216.
J. Dongun Kim Stefan Senn Arye Harel Benjamin I. Jelen Paul G. Falkowski 《Philosophical transactions of the Royal Society of London. Series B, Biological sciences》2013,368(1622)
Oxidoreductases play a central role in catalysing enzymatic electron-transfer reactions across the tree of life. To first order, the equilibrium thermodynamic properties of these proteins are governed by protein folds associated with specific transition metals and ligands at the active site. A global analysis of holoenzyme structures and functions suggests that there are fewer than approximately 500 fundamental oxidoreductases, which can be further clustered into 35 unique groups. These catalysts evolved in prokaryotes early in the Earth''s history and are largely responsible for the emergence of non-equilibrium biogeochemical cycles on the planet''s surface. Although the evolutionary history of the amino acid sequences in the oxidoreductases is very difficult to reconstruct due to gene duplication and horizontal gene transfer, the evolution of the folds in the catalytic sites can potentially be used to infer the history of these enzymes. Using a novel, yet simple analysis of the secondary structures associated with the ligands in oxidoreductases, we developed a structural phylogeny of these enzymes. The results of this ‘composome’ analysis suggest an early split from a basal set of a small group of proteins dominated by loop structures into two families of oxidoreductases, one dominated by α-helices and the second by β-sheets. The structural evolutionary patterns in both clades trace redox gradients and increased hydrogen bond energy in the active sites. The overall pattern suggests that the evolution of the oxidoreductases led to decreased entropy in the transition metal folds over approximately 2.5 billion years, allowing the enzymes to use increasingly oxidized substrates with high specificity. 相似文献
217.
Young Man Park Kazuya Matsumoto Yoo Jin Seo Min Jeung Kang Hidetoshi Nagashima 《Biological Rhythm Research》2013,44(1):39-51
The healthy 455 subjects above 60 years of age were questioned on their sleep habit inventory and the morningness-eveningness questionnaire. We analyzed the effects of age and sex on sleep habits and sleep-related trouble. Bedtimes on weekdays and weekends became earlier with aging, and women went to bed significantly later than men did. The length of sleep on weekdays slightly increased with aging, and it was longer for men than for women. The number of urinations and awakenings during nocturnal sleep and the amount of daytime napping increased with aging. The score on morningness-eveningness shifted toward the morning type with aging. In comparison with men, women had significantly longer sleep latency; and a higher percentage of subjects who reported that they sleep for only a short time, have sleep trouble, have received medical treatment for their sleep trouble, and take sleep medication. From these results, we deduced that the phase of sleep shifted forward in subjects above 60 years of age, and they showed frequent interruptions during nocturnal sleep and long daytime napping. We discussed the factor of gender difference in sleep in relation to social and cultural factors, particularly the household activities of women. 相似文献
218.
C.R.C. Moreno L. Matuzaki F. Carvalho R. Alves I. Pasqua G. Lorenzi-Filho 《Biological Rhythm Research》2013,44(2):137-143
Irregular working hours, including night work, change sleep-wake time arrangements which in turn might affect the ability to drive safely. This study aims to compare the effects of an irregular and a fixed day shift system on the sleep-wake cycle of truck drivers. The investigation of sleep-wake cycle was carried-out with 37 truck drivers working on two transportation plants: 24 working on irregular working hours and 13 on fixed day shift. The truck drivers filled out sleep logs and wore actigraphs for 10 consecutive days to identify activity and rest episodes. The group working in irregular hours showed more sleep episodes per 24 h and they were shorter compared to the fixed shift group (p < 0.05). No differences were found between the two transportation plants. These results suggest an the influence of working hours on specific sleep-wake patterns. The polyphasic sleep pattern shown by irregular shift group could be a strategy to cope with sleep deprivation, which may account for their difficulty to resist falling asleep behind the wheel. 相似文献
219.
220.
Abstract Sexually mature male and female Rana esculenta L. were captured in their natural habitat in six phases of the annual cycle. Nuclear volumes in APOA cells were found to fluctuate distinctly in the course of the year. In both sexes nuclear volumes were maximal in the phases preceding the breeding season (IIIrd decade of January, and 1st decade of April), and minimal throughout the phases of active life (IIIrd decade of May, IInd decade of July, and 1st decade of September). No aldehydefuchsin or Gomori‐positive material was found in the APOA perikaryons. 相似文献