全文获取类型
收费全文 | 15615篇 |
免费 | 1134篇 |
国内免费 | 2篇 |
专业分类
林业 | 1524篇 |
农学 | 568篇 |
基础科学 | 70篇 |
3730篇 | |
综合类 | 1047篇 |
农作物 | 672篇 |
水产渔业 | 1124篇 |
畜牧兽医 | 6183篇 |
园艺 | 374篇 |
植物保护 | 1459篇 |
出版年
2023年 | 157篇 |
2022年 | 179篇 |
2021年 | 354篇 |
2020年 | 365篇 |
2019年 | 256篇 |
2018年 | 682篇 |
2017年 | 657篇 |
2016年 | 641篇 |
2015年 | 525篇 |
2014年 | 617篇 |
2013年 | 977篇 |
2012年 | 1247篇 |
2011年 | 1170篇 |
2010年 | 626篇 |
2009年 | 495篇 |
2008年 | 982篇 |
2007年 | 911篇 |
2006年 | 765篇 |
2005年 | 765篇 |
2004年 | 687篇 |
2003年 | 657篇 |
2002年 | 543篇 |
2001年 | 351篇 |
2000年 | 320篇 |
1999年 | 265篇 |
1998年 | 92篇 |
1997年 | 77篇 |
1996年 | 63篇 |
1995年 | 71篇 |
1994年 | 56篇 |
1993年 | 58篇 |
1992年 | 96篇 |
1991年 | 80篇 |
1990年 | 72篇 |
1989年 | 73篇 |
1988年 | 71篇 |
1987年 | 52篇 |
1986年 | 50篇 |
1985年 | 50篇 |
1984年 | 49篇 |
1983年 | 39篇 |
1981年 | 27篇 |
1979年 | 39篇 |
1974年 | 31篇 |
1973年 | 22篇 |
1972年 | 22篇 |
1971年 | 26篇 |
1970年 | 29篇 |
1969年 | 29篇 |
1968年 | 30篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
101.
Metlholo Andries Phukuntsi Helene Brettschneider Desiré L Dalton Teresa Kearney Jacqueline Badenhorst Antoinette Kotze 《African Zoology》2016,51(1):69-76
DNA barcoding has been proposed as a method for species identification. However, this method has been criticised for its over-reliance on a single mitochondrial gene. In this study, four mitochondrial gene regions and one nuclear gene region were used to investigate their different abilities to identify tissue associated with museum specimens of Aethomys chrysophilus, Aethomys ineptus and Micaelamys namaquensis. Aethomys chrysophilus and the more recently elevated A. ineptus are indistinguishable on morphological grounds; however, their ranges are largely parapatric with only one syntopic locality currently known. All of the mitochondrial gene regions were able to separate M. namaquensis from A. chrysophilus and A. ineptus, but they varied in their abilities to resolve differences between A. chrysophilus and A. ineptus. The sequence results identified a specimen from KwaZulu-Natal that was misclassified and should have been identified as A. ineptus. Seven specimens that had not been reclassified following the elevation of A. ineptus to species level were identified as A. ineptus. Individuals of A. chrysophilus from Malawi could not be classified as either A. chrysophilus or A. ineptus, and may be a hybrid or a new, distinct species. This study indicates that DNA barcoding may be used to separate M. namaquensis from A. chrysophilus and A. ineptus, and although it was not able to separate A. chrysophilus and A. ineptus, it did indicate specimens from Malawi may be a new cryptic species. 相似文献
102.
The African Penguin (Spheniscus demersus) has suffered population declines and is listed in the IUCN Red List as Endangered. The species is endemic to the coast of southern Africa, and breeding colonies are distributed on the south-western coast of Africa. Currently, African Penguins are being kept in zoo and aquarium facilities throughout South Africa. In this study, molecular genetic data based on 12 microsatellite markers from 1 119 African Penguin samples from four facilities were generated in order to determine the level of genetic variation, population structure and differentiation, and effective population size to assist in the development of an effective captive management plan. Expected heterozygosity ranged from 0.57 to 0.62, and allelic richness from 4.2 to 5.1. However, based on differences between first- and second-generation captive birds, we conclude that the ex situ population is at risk of losing genetic variability in the future and management programmes should include exchange of birds between captive facilities in order to induce gene flow and increase effective population size. Adding individuals from in situ populations should also be considered in the future in cases where these birds cannot be rehabilitated. Molecular genetic analyses of wild penguin populations should be carried out for comparison, and to ascertain to what degree ‘in situ genetic diversity’ is represented among ex situ populations. With regular resampling and analyses, the extent of the effect of processes such as genetic drift on diversity in the ex situ penguin populations will become evident. 相似文献
103.
Fabien Génin Ayabulela Yokwana Nokuthula Kom Sébastien Couette Thibault Dieuleveut Stephen D Nash 《African Zoology》2016,51(3):135-143
The primate fauna of South Africa has historically been viewed as comprising three diurnal cercopithecoid taxa – chacma baboons (Papio ursinus), vervet (Chlorocebus pygerythrus) and samango monkeys (Cercopithecus albogularis) – and two nocturnal lorisoid species – the thick-tailed greater galago (Otolemur crassicaudatus) and the southern lesser galago (Galago moholi). Here we report the positive identification of a third galago species within South Africa’s borders: the Mozambique dwarf galago or Grant’s galago, Galagoides granti (Thomas and Wroughton, 1907). The taxon was previously held to be restricted to Mozambique, eastern Zimbabwe, Malawi and Tanzania, but we have also observed it in the sand forest of Tembe Elephant Park and the Tshanini Community Reserve, near the Mozambique border. The species was formerly mistaken for Galago moholi, erroneously (we believe) extending the range of the latter species into northern KwaZulu-Natal. In South Africa the two small galagos are unlikely to have overlapping ranges: Galago moholi prefers dry savanna woodlands, whereas Galagoides granti is apparently confined to dry sand forest. However, both species may coexist with the larger and more widespread Otolemur crassicaudatus, an inhabitant of moist savanna, forest edge and thicket. The true South African ranges of both small galago species need to be ascertained. 相似文献
104.
In Tunisia both Psammomys obesus and P. vexillaris are found. These taxa have been the subject of taxonomic controversy for some time, due to variability in the classical morphological characters used for taxonomic recognition. In this study we investigated skull size and shape variation in the genus Psammomys by using geometric morphometrics to evaluate the extent of intra- and interspecific shape variation and explicitly tested for the impact of allometric shape variation on species discrimination. Eleven populations of the two species from 10 localities in Tunisia were studied. Statistical analyses of size and shape showed large size variation within P. obesus, but no shape differences were revealed among populations of this species. Interspecific analysis revealed that P. vexillaris had the smallest skull. Principal component analysis and Procrustes distances showed good discrimination between the two species after removal of the allometric component of shape variation. The results obtained show that allometric-related shape variation could mask discrimination between the two Psammomys species. This finding might explain the uncertainty in classification of these species in the past. The interspecific allometric-free phenotypic differences observed may be associated with adaptive processes linked to the different environmental and trophic preferences of the two species. 相似文献
105.
Comparison of 4 point‐of‐care blood gas analyzers for arterial blood gas analysis in healthy dogs and dogs with cardiopulmonary disease 下载免费PDF全文
106.
107.
108.
109.
110.
C. Ekstrand C. Ingvast‐Larsson L. Olsén M. Hedeland U. Bondesson J. Gabrielsson 《Journal of veterinary pharmacology and therapeutics》2016,39(3):255-263
The cortisol response to glucocorticoid intervention has, in spite of several studies in horses, not been fully characterized with regard to the determinants of onset, intensity and duration of response. Therefore, dexamethasone and cortisol response data were collected in a study applying a constant rate infusion regimen of dexamethasone (0.17, 1.7 and 17 μg/kg) to six Standardbreds. Plasma was analysed for dexamethasone and cortisol concentrations using UHPLC‐MS/MS. Dexamethasone displayed linear kinetics within the concentration range studied. A turnover model of oscillatory behaviour accurately mimicked cortisol data. The mean baseline concentration range was 34–57 μg/L, the fractional turnover rate 0.47–1.5 1/h, the amplitude parameter 6.8–24 μg/L, the maximum inhibitory capacity 0.77–0.97, the drug potency 6–65 ng/L and the sigmoidicity factor 0.7–30. This analysis provided a better understanding of the time course of the cortisol response in horses. This includes baseline variability within and between horses and determinants of the equilibrium concentration–response relationship. The analysis also challenged a protocol for a dexamethasone suppression test design and indicated future improvement to increase the predictability of the test. 相似文献