全文获取类型
收费全文 | 2490篇 |
免费 | 83篇 |
国内免费 | 14篇 |
出版年
2015年 | 14篇 |
2014年 | 15篇 |
2013年 | 59篇 |
2012年 | 59篇 |
2011年 | 50篇 |
2010年 | 65篇 |
2009年 | 111篇 |
2008年 | 79篇 |
2007年 | 62篇 |
2006年 | 58篇 |
2005年 | 54篇 |
2004年 | 51篇 |
2003年 | 25篇 |
2002年 | 16篇 |
2001年 | 16篇 |
2000年 | 16篇 |
1999年 | 30篇 |
1998年 | 39篇 |
1997年 | 43篇 |
1996年 | 39篇 |
1995年 | 27篇 |
1994年 | 24篇 |
1993年 | 21篇 |
1992年 | 35篇 |
1990年 | 22篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 21篇 |
1986年 | 21篇 |
1985年 | 20篇 |
1984年 | 15篇 |
1983年 | 20篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 21篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1971年 | 16篇 |
1959年 | 49篇 |
1958年 | 123篇 |
1957年 | 137篇 |
1956年 | 134篇 |
1955年 | 149篇 |
1954年 | 150篇 |
1953年 | 119篇 |
1952年 | 104篇 |
1951年 | 98篇 |
1950年 | 59篇 |
1949年 | 14篇 |
1948年 | 23篇 |
排序方式: 共有2587条查询结果,搜索用时 984 毫秒
201.
MAURO GOBBI BRUNO ROSSARO AMBER VATER FIORENZA DE BERNARDI MANUELA PELFINI PIETRO BRANDMAYR 《Ecological Entomology》2007,32(6):682-689
Abstract. 1. The spatio-temporal approach was used to evaluate the environmental features influencing carabid beetle assemblages along a chronosequence of an Italian Alpine glacier foreland. The influence of environmental variables on species richness, morphology (wing and body length), and distribution along the chronosequence was tested.
2. Species richness was found to be a poor indicator of habitat due to weak influences by environmental variables. It seems that the neighbouring habitats of a glacier foreland are not able to determine significant changes in carabid species richness.
3. Instead it appears that history (age since deglaciation) and habitat architecture of a glacier foreland are strongly correlated to species adaptive morphological traits, such as wing morphology and body length. Assemblages characterised by species with reduced wing size are linked to the older stages of the chronosequence, where habitat is more structured. Assemblages characterised by the largest species are linked to the younger sites near the glacier. These morphological differentiations are explained in detail.
4. Habitat age can therefore be considered the main force determining assemblage composition. On the basis of the relationship between morphological traits and environmental variables, it seems likely that age since deglaciation is the main variable influencing habitat structure (primary effect) on the Forni foreland. The strong relationship between carabid assemblages and habitat type indicates that site age has but a secondary effect on carabid assemblages. This may be utilised to interpret potential changes in assemblages linked to future glacier retreat. 相似文献
2. Species richness was found to be a poor indicator of habitat due to weak influences by environmental variables. It seems that the neighbouring habitats of a glacier foreland are not able to determine significant changes in carabid species richness.
3. Instead it appears that history (age since deglaciation) and habitat architecture of a glacier foreland are strongly correlated to species adaptive morphological traits, such as wing morphology and body length. Assemblages characterised by species with reduced wing size are linked to the older stages of the chronosequence, where habitat is more structured. Assemblages characterised by the largest species are linked to the younger sites near the glacier. These morphological differentiations are explained in detail.
4. Habitat age can therefore be considered the main force determining assemblage composition. On the basis of the relationship between morphological traits and environmental variables, it seems likely that age since deglaciation is the main variable influencing habitat structure (primary effect) on the Forni foreland. The strong relationship between carabid assemblages and habitat type indicates that site age has but a secondary effect on carabid assemblages. This may be utilised to interpret potential changes in assemblages linked to future glacier retreat. 相似文献
202.
CYRIL BONTEMPS GEOFFREY N. ELLIOTT† MARCELO F. SIMON‡ FÁBIO B. DOS REIS JÚNIOR § EDUARDO GROSS¶ REBECCA C. LAWTON NICOLAU ELIAS NETO MARIA de FÁTIMA LOUREIRO SERGIO M. DE FARIA†† JANET I. SPRENT‡‡ EUAN K. JAMES§§ J. PETER W. YOUNG 《Molecular ecology》2010,19(1):44-52
Burkholderia has only recently been recognized as a potential nitrogen-fixing symbiont of legumes, but we find that the origins of symbiosis in Burkholderia are much deeper than previously suspected. We sampled 143 symbionts from 47 native species of Mimosa across 1800 km in central Brazil and found that 98% were Burkholderia . Gene sequences defined seven distinct and divergent species complexes within the genus Burkholderia . The symbiosis-related genes formed deep Burkholderia -specific clades, each specific to a species complex, implying that these genes diverged over a long period within Burkholderia without substantial horizontal gene transfer between species complexes. 相似文献
203.
We investigated the influence of sampling location within a faeces on DNA quality by sampling from both the outside and inside of 25 brown bear (Ursus arctos) scats and the side and the tip of 30 grey wolf (Canis lupus) scats. The outside of the bear scat and side of the wolf scat had significantly lower nuclear DNA microsatellite allelic dropout error rates (U. arctos: P = 0.017; C. lupus: P = 0.025) and significantly higher finalized genotyping success rates (U. arctos: P = 0.017; C. lupus: P = 0.012) than the tip and inside of the scat. A review of the faecal DNA literature indicated that <45% of studies report the sampling location within a faeces indicating that this methodological consideration is currently underappreciated. Based on our results, we recommend sampling from the side of canid scats and the outside portion of ursid scats to obtain higher quality DNA samples. The sampling location within a faeces should be carefully considered and reported as it can directly influence laboratory costs and efficiency, as well as the ability to obtain reliable genotypes. 相似文献
204.
CARLOS DE ANGELO AGUSTÍN PAVIOLO MARIO S. DI BITETTI 《The Journal of wildlife management》2010,74(5):1141-1151
ABSTRACT The jaguar (Panthera onca) and puma (Puma concolor) are the largest felids of the American Continent and live in sympatry along most of their distribution. Their tracks are frequently used for research and management purposes, but tracks are difficult to distinguish from each other and can be confused with those of big canids. We used tracks from pumas, jaguars, large dogs, and maned wolves (Chrysocyon brachyurus) to evaluate traditional qualitative and quantitative identification methods and to elaborate multivariate methods to differentiate big canids versus big felids and puma versus jaguar tracks (n = 167 tracks from 18 zoos). We tested accuracy of qualitative classification through an identification exercise with field-experienced volunteers. Qualitative methods were useful but there was high variability in accuracy of track identification. Most of the traditional quantitative methods showed an elevated percentage of misclassified tracks (≥20%). We used stepwise discriminant function analysis to develop 3 discriminant models: 1 for big canid versus big felid track identification and 2 alternative models for jaguar versus puma track differentiation using 1) best discriminant variables, and 2) size-independent variables. These models had high classification performance, with <10% of error in the validation procedures. We used simpler discriminant models in the elaboration of identification keys to facilitate track classification process. We developed an accurate method for track identification, capable of distinguishing between big felids (puma and jaguar) and large canids (dog and maned wolf) tracks and between jaguar and puma tracks. Application of our method will allow a more reliable use of tracks in puma and jaguar research and it will help managers using tracks as indicators of these felids' presence for conservation or management purposes. 相似文献
205.
206.
207.
PATRICIA FERNANDES NERES VERÔNICA GOMES DA FONSECA‐GENEVOIS RODRIGO AUGUSTO TORRES MARIANA DA FONSECA CAVALCANTI FRANCISCO JOSÉ VICTOR DE CASTRO NEYVAN RENATO RODRIGUES DA SILVA TANIA TASSINARI RIEGER WILFRIDA DECRAEMER 《Zoological Journal of the Linnean Society》2010,158(1):1-15
A new species of Daptonema is described based upon morphological characters and 18S rRNA sequence. Daptonema matrona sp. nov. was collected in Pina Basin (north‐eastern Brazil). It differs from all other species of the genus by the presence of reduced cephalic setae and straight spicules. These features require an adaptation of the generic diagnosis. Moreover, the females are characterized by intra‐uterine development of the offspring, considered herein as their major autapomorphic feature. Molecular systematic analyses supported Daptonema matrona sp. nov. as a distinct genetic and evolutionary lineage. The data also indicate hypotheses of taxonomic synonymies amongst some related taxa from Xyalidae as well as the paraphyly of Daptonema. © 2010 The Linnean Society of London, Zoological Journal of the Linnean Society, 2010, 158 , 1–15. 相似文献
208.
DALIBOR KODRÍK HEATHER G. MARCO PETR ŠIMEK RADOMÍR SOCHA PAVEL ŠTYS GERD GÄDE 《Physiological Entomology》2010,35(2):117-127
The adipokinetic hormones (AKHs) from 15 species of heteropteran Hemiptera (encompassing eight families, six superfamilies and three infraorders) have been isolated and structurally identified using liquid chromatography coupled with mass spectrometry. None of the structures are novel and all are octapeptides. These peptide sequence data are used, together with the previously available AKH sequence data on Heteroptera, to create a larger dataset for comparative analyses. This results, in total, in AKH sequences from 30 species (spanning 13 families), which are used in a matrix confronted with the current hypotheses on the phylogeny of Heteroptera. The expanded dataset shows that all heteropterans have octapeptide AKHs; three species have two AKHs, whereas the overwhelming majority have only one AKH. From a total of 11 different AKH peptides known from Heteroptera to date, three AKHs occur frequently: Panbo‐red pigment‐concentrating hormone (RPCH) (×10), Schgr‐AKH‐II (×6) and Anaim‐AKH (×4). The heteropteran database also suggests that particular AKH variants are family‐specific. The AKHs of Heteroptera: Pentatomomorpha (all terrestrial) are not present in Nepomorpha (aquatic) and Gerromorpha: Gerridae (semiaquatic); AKHs with a Val in position 2 are absent in the Pentatomomorpha (only AKHs with Leu2 are present), whereas Val2 predominates in the nonterrestrial species. An unexpected diversity of AKH sequences is found in Nepomorpha, Nepoidea, Nepidae and Nepinae, whereas Panbo‐RPCH (which has been identified in all infraorders of decapod crustaceans) is present in all analysed species of Pentatomidae and also in the only species of Tessaratomidae investigated. The molecular evolution of Heteroptera with respect to other insect groups and to crustaceans is discussed 相似文献
209.
INMACULADA DE MELO-MARTÍN 《Bioethics》2006,20(6):301-307
The purpose of this paper is to show that a decontextualized approach to ethical issues is not just unhelpful for the decision making process of real, situated human beings, but dangerous. This is so, because by neglecting the context in which people make moral decisions we run the risk of reinforcing or furthering injustices against already disadvantaged groups. To show this, I evaluate three moral obligations that our ability to obtain genetic information has made salient: the duty to obtain genetic information about ourselves, the obligation to inform family members about genetic risks and the duty not to reproduce when we know that there is a high risk of transmitting a serious disease or defect. I will argue here that in ignoring the context in which these moral obligations are put into practice, and in particular the situation of women in our society, those who defend these moral duties might be furthering injustices against women. 相似文献
210.
VERONICA DE MICCO GIOVANNA ARONNE 《Botanical journal of the Linnean Society. Linnean Society of London》2007,155(3):361-371
Mediterranean shrub species are described as having phenology, habitus , reproductive biology and anatomical alterations in certain tissues, allowing their survival during the dry season and protecting them from herbivory. Anatomical and chemical analyses were conducted in 1-year-old branches of Cistus ladanifer L. in order to investigate the role played by shoot structure in the adaptive strategies of this species in the Mediterranean environment. Results showed that both xylem and pith underwent lignification. Pith parenchyma cells had thickened walls, higher lignin content than xylem and different monomer composition. Xylem presented features aiding safe water transport. A large accumulation of phenolic substances was found in xylem, pith and cortical parenchyma. Observations reported in this paper suggest the occurrence of adaptive strategies in 1-year-old branches of C. ladanifer whose structural features: (1) allow mechanical reinforcement of tissues to withstand drought without suffering permanent damage; (2) favour safety rather than efficiency in water transport; (3) defend the plants from animal predation and pathogens by accumulating phenolics in various tissues, and (4) protect inner tissues against UV-B radiation through deposition of phenolic compounds in cortical layers. © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society, 2007, 155 , 361–371. 相似文献