全文获取类型
收费全文 | 44952篇 |
免费 | 16832篇 |
国内免费 | 2452篇 |
出版年
2024年 | 27篇 |
2023年 | 231篇 |
2022年 | 387篇 |
2021年 | 1217篇 |
2020年 | 2753篇 |
2019年 | 4421篇 |
2018年 | 4469篇 |
2017年 | 4615篇 |
2016年 | 4905篇 |
2015年 | 5137篇 |
2014年 | 5093篇 |
2013年 | 5575篇 |
2012年 | 3580篇 |
2011年 | 3153篇 |
2010年 | 4106篇 |
2009年 | 2736篇 |
2008年 | 1888篇 |
2007年 | 1347篇 |
2006年 | 1248篇 |
2005年 | 1211篇 |
2004年 | 1112篇 |
2003年 | 1033篇 |
2002年 | 949篇 |
2001年 | 623篇 |
2000年 | 485篇 |
1999年 | 406篇 |
1998年 | 248篇 |
1997年 | 211篇 |
1996年 | 166篇 |
1995年 | 121篇 |
1994年 | 96篇 |
1993年 | 73篇 |
1992年 | 100篇 |
1991年 | 75篇 |
1990年 | 53篇 |
1989年 | 47篇 |
1988年 | 40篇 |
1987年 | 42篇 |
1986年 | 18篇 |
1985年 | 29篇 |
1984年 | 25篇 |
1983年 | 15篇 |
1982年 | 12篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 15篇 |
1970年 | 10篇 |
1968年 | 9篇 |
1966年 | 9篇 |
1965年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
After observing specimens of Calligonum pumilum Losinsk. and C. juochiangense Y. X. Liou in both the field and in herbarium collections, it was found that the morphological characters of these two species are quite different, especially with respect of the twisted direction of fruit ribs, number of bristle rows along each rib, rigidity and degree of interweaving of bristles, as well as their geographic distribution. Therefore, it is concluded that C. pumilum and C. juochiangense should be accepted as two independent species. 相似文献
102.
103.
T. RIGOT M. VERCAUTEREN DRUBBEL J.‐C. DELÉCOLLE M. GILBERT 《Medical and veterinary entomology》2013,27(1):29-38
The spatial epidemiology of Bluetongue virus (BTV) at the landscape level relates to the fine‐scale distribution and dispersal capacities of its vectors, midges belonging to the genus Culicoides Latreille (Diptera: Ceratopogonidae). Although many previous researches have carried out Culicoides sampling on farms, little is known of the fine‐scale distribution of Culicoides in the landscape immediately surrounding farms. The aim of this study was to gain a better understanding of Culicoides populations at increasing distances from typical dairy farms in north‐west Europe, through the use of eight Onderstepoort‐type black‐light traps positioned along linear transects departing from farms, going through pastures and entering woodlands. A total of 16 902 Culicoides were collected in autumn 2008 and spring 2009. The majority were females, of which more than 97% were recognized as potential vectors. In pastures, we found decreasing numbers of female Culicoides as a function of the distance to the farm. This pattern was modelled by leptokurtic models, with parameters depending on season and species. By contrast, the low number of male Culicoides caught were homogeneously distributed along the transects. When transects entered woodlands, we found a higher abundance of Culicoides than expected considering the distance of the sampling sites to the farm, although this varied according to species. 相似文献
104.
105.
106.
107.
108.
Distinct effects of climate warming on populations of silver fir (Abies alba) across Europe 下载免费PDF全文
109.
由于圆锥角膜疾病导致越来越多的人患有近视,常见的矫正方法有佩戴近视眼镜、隐形眼镜等.随着科技的进步,利用光对近视等眼科疾病进行屈光矫正已经成为当前临床中常用的方法.使用光诱导角膜胶原蛋白发生交联,从而达到治疗圆锥角膜疾病、提高患者视力水平的目的,这是一种新型的光治疗眼睛疾病的方法.同时这种方法由于无侵入性、对操作者能力依赖性小等优势成为新的研究热点.本文阐述光诱导角膜交联的基本原理,并介绍其发展历程,分析现有的各种交联方法和角膜检测技术的原理,并对现有交联方法和检测方法的优缺点进行讨论.最后,本文对光诱导角膜交联和检测技术的最新进展进行系统的论述,并对未来的发展趋势进行展望. 相似文献
110.
Integrated species delimitation and conservation implications of an endangered weevil Pachyrhynchus sonani (Coleoptera: Curculionidae) in Green and Orchid Islands of Taiwan 下载免费PDF全文
YEN‐TING CHEN HUI‐YUN TSENG MING‐LUEN JENG YONG‐CHAO SU WEN‐SAN HUANG CHUNG‐PING LIN 《Systematic Entomology》2017,42(4):796-813
Oceanic islands are productive habitats for generating new species and high endemism, which is primarily due to their geographical isolation, smaller population sizes and local adaptation. However, the short divergence times and subtle morphological or ecological divergence of insular organisms may obscure species identity, so the cryptic endemism on islands may be underestimated. The endangered weevil Pachyrhynchus sonani Kôno (Coleoptera: Curculionidae: Entiminae: Pachyrhynchini) is endemic to Green Island and Orchid Island of the Taiwan‐Luzon Archipelago and displays widespread variation in coloration and host range, thus raising questions regarding its species boundaries and degree of cryptic diversity. We tested the species boundaries of P. sonani using an integrated approach that combined morphological (body size and shape, genital shape, coloration and cuticular scale), genetic (four genes and restriction site‐associated DNA sequencing, RAD‐seq) and ecological (host range and distribution) diversity. The results indicated that all the morphological datasets for male P. sonani, except for the colour spectrum, reveal overlapping but statistically significant differences between islands. In contrast, the morphology of the female P. sonani showed minimum divergence between island populations. The populations of P. sonani on the two islands were significantly different in their host ranges, and the genetic clustering and phylogenies of P. sonani established two valid evolutionary species. Integrated species delimitation combining morphological, molecular and ecological characters supported two distinct species of P. sonani from Green Island and Orchid Island. The Green Island population was described as P. jitanasaius sp.n. Chen & Lin, and it is recommended that its threatened conservation status be recognized. Our findings suggest that the inter‐island speciation of endemic organisms inhabiting both islands may be more common than previously thought, and they highlight the possibility that the cryptic diversity of small oceanic islands may still be largely underestimated. 相似文献