全文获取类型
收费全文 | 8212篇 |
免费 | 674篇 |
国内免费 | 519篇 |
专业分类
9405篇 |
出版年
2024年 | 30篇 |
2023年 | 112篇 |
2022年 | 128篇 |
2021年 | 220篇 |
2020年 | 288篇 |
2019年 | 267篇 |
2018年 | 250篇 |
2017年 | 244篇 |
2016年 | 289篇 |
2015年 | 320篇 |
2014年 | 385篇 |
2013年 | 605篇 |
2012年 | 323篇 |
2011年 | 338篇 |
2010年 | 251篇 |
2009年 | 355篇 |
2008年 | 397篇 |
2007年 | 358篇 |
2006年 | 347篇 |
2005年 | 319篇 |
2004年 | 262篇 |
2003年 | 299篇 |
2002年 | 267篇 |
2001年 | 228篇 |
2000年 | 190篇 |
1999年 | 180篇 |
1998年 | 183篇 |
1997年 | 179篇 |
1996年 | 152篇 |
1995年 | 150篇 |
1994年 | 117篇 |
1993年 | 139篇 |
1992年 | 122篇 |
1991年 | 111篇 |
1990年 | 99篇 |
1989年 | 121篇 |
1988年 | 85篇 |
1987年 | 75篇 |
1986年 | 80篇 |
1985年 | 59篇 |
1984年 | 77篇 |
1983年 | 57篇 |
1982年 | 56篇 |
1981年 | 59篇 |
1980年 | 44篇 |
1979年 | 54篇 |
1978年 | 35篇 |
1977年 | 31篇 |
1976年 | 21篇 |
1973年 | 14篇 |
排序方式: 共有9405条查询结果,搜索用时 0 毫秒
961.
Mehmet Yaar can 《American journal of physical anthropology》1989,78(1):3-8
This study analyzes and compares the dental size variation and sexual dimorphism of the Highland Beach Mound people, a late southeastern prehistoric aboriginal Florida population (ca. 800-1200 AD) with a somewhat earlier midwestern group (ca. 200-800 AD) and an Archaic sample (ca. 4160-2558 BC). The lengths and breadths of maxillary and mandibular permanent teeth of 99 individuals were measured, and crown surface areas were calculated for the molars. Results indicated that there was little sexual dimorphism. Only the mesiodistal dimensions of the upper P2 and lower C were significantly different at the P less than 0.05 level. In terms of dental size, what appears to be a chronologically based reduction in most points of comparison was seen in all three groups, with the exception of the molars, which were larger in the Highland Beach sample than in the earlier Schultz Mound. 相似文献
962.
Comparative egg morphology of six Meccus species and Triatoma recurva (Stål, 1868) Hemiptera: Reduviidae 下载免费PDF全文
Nancy Rivas Esther Sánchez‐Espíndola Alejandro D. Camacho Ricardo Alejandre‐Aguilar 《Journal of vector ecology》2016,41(1):135-141
We investigated the morphology and morphometry of eggs from the colonies of the Entomology Laboratory at ENCB‐IPN belonging to six species of the genus Meccus and Triatoma recurva, using both light and scanning electron microscopy. Egg ornamentation is an important parameter to consider in the differentiation of species. Samples were observed under a scanning electron microscope. Measurements of fifty eggs per species included length, width, and opercular diameter, which were used for the morphometric analysis. The results showed that the seven species display a polygonal ornamentation dominated by hexagons; the operculum shows also a polygonal ornamental characteristic in each species. Morphometry provided valuable information for discriminating between closely related species of the genus Meccus and Triatoma recurva, a species akin to this group, thereby facilitating the complete discrimination of these species. 相似文献
963.
Schulte-Hostedde AI Kuula S Martin C Schank CC Lesbarrères D 《Journal of evolutionary biology》2011,24(5):1154-1159
Allometry of secondary sexual traits has been the subject of recent debate, and the generality of positive allometry and its association with sexual selection have been recently questioned. Whereas some studies suggest an almost universal positive allometry for traits under sexual selection and isometry or a negative allometry for traits not under such pressure, other studies argue that this pattern results from the study of exaggerated (ornamental) traits. To answer the call for an examination of the allometry of less-exaggerated sexually selected traits, we have examined morphological data from 14 sexually dimorphic traits and six monomorphic traits from three anuran species. Although we found evidence of positive allometry in male secondary sexual traits of several species and populations, not all nonsexual traits were isometric or exhibited negative allometry. Furthermore, our results indicate that larger traits in the populations that we studied were not associated with greater allometric slopes. Therefore, our study is in line with the contention suggesting no specific kind of allometric pattern for sexual and nonsexual characters, and we can only advocate for further investigation of trait allometry and sexual selection to understand the complexity underlying the evolution of allometry in sexual traits. 相似文献
964.
Integrative taxonomy of Parasabella and Sabellomma (Sabellidae: Annelida) from Australia: description of new species,indication of cryptic diversity,and translocation of some species out of their natural distribution range 下载免费PDF全文
Most nominal fan worm species belonging to the genus Parasabella Bush, 1905, and Sabellomma Nogueira, Fitzhugh & Silva‐Rossi, 2010 (Sabellidae, Annelida) lack unique morphological diagnostic features. Species diagnoses rely on a combination of attributes, often making reference to characters that are qualitatively or quantitatively continuous. This, together with a lack of knowledge about phenotypic intraspecific variation in most species, suggests that the sole use of morphological features is insufficient to identify and characterize some of these species. Examination of collections revealed that Parasabella is a common and diverse group of sabellids along the Australian coastline, although it had only been recorded here twice. The genus Sabellomma is here reported in Australia for the first time. Comparison of morphological data and nuclear (internal transcribed spacer) and mitochondrial (cytochrome oxidase I) DNA sequence data of 30 terminals in a range of analyses allowed us to assess the monophyly of Parasabella and its sister‐group relationship with Sabellomma and to examine some of the species boundaries and genetic diversity within lineages. The combined sequence data results indicate the presence of seven distinct genetic Parasabella lineages in Australia, four of which are assigned to previously described species (three are new records for Australia), and two are considered as a complex of species sharing a unique combination of characters, one of which is described as new. Another Parasabella species with distinct atypical radiolar eyes and a species of Sabellomma are also described as new. © 2015 The Linnean Society of London 相似文献
965.
966.
DNA barcoding largely supports 250 years of classical taxonomy: identifications for Central European bees (Hymenoptera,Apoidea partim) 下载免费PDF全文
Stefan Schmidt Christian Schmid‐Egger Jérôme Morinière Gerhard Haszprunar Paul D. N. Hebert 《Molecular ecology resources》2015,15(4):985-1000
This study presents DNA barcode records for 4118 specimens representing 561 species of bees belonging to the six families of Apoidea (Andrenidae, Apidae, Colletidae, Halictidae, Megachilidae and Melittidae) found in Central Europe. These records provide fully compliant barcode sequences for 503 of the 571 bee species in the German fauna and partial sequences for 43 more. The barcode results are largely congruent with traditional taxonomy as only five closely allied pairs of species could not be discriminated by barcodes. As well, 90% of the species possessed sufficiently deep sequence divergence to be assigned to a different Barcode Index Number (BIN). In fact, 56 species (11%) were assigned to two or more BINs reflecting the high levels of intraspecific divergence among their component specimens. Fifty other species (9.7%) shared the same Barcode Index Number with one or more species, but most of these species belonged to a distinct barcode cluster within a particular BIN. The barcode data contributed to clarifying the status of nearly half the examined taxonomically problematic species of bees in the German fauna. Based on these results, the role of DNA barcoding as a tool for current and future taxonomic work is discussed. 相似文献
967.
The role of domestication and maternal effects on seed traits of crop–wild sunflower hybrids (Helianthus annuus) 下载免费PDF全文
F. Hernández L.I. Lindström E. Parodi M. Poverene A. Presotto 《The Annals of applied biology》2017,171(2):237-251
Hybridisation between crops and their wild relatives may promote the evolution of weeds. Seed germination and dormancy are the earliest life‐history traits and are highly influenced by the maternal parent. However, the ecological role of the maternal effect on seed traits in the evolution of crop–wild hybrids has received little attention. In this study, we test the relative importance of maternal and hybridisation effects on seed traits of the first generation of crop–wild sunflower hybrids (Helianthus annuus). Seed germination was tested in two wild populations with contrasting dormancy, two cultivated materials and their reciprocal crosses at four different times after harvest and three different temperatures. Seed germination at each of the four times, after ripening response and secondary dormancy were recorded along with four morphological traits. Additionally, the pericarp anatomy was analysed with light and scanning electron microscopy. We observed strong maternal effects on all seed traits. Seed germination, morphology and pericarp anatomy differed largely between the crop and wild seeds and these traits in the crop–wild hybrids resembled their female parent. Slight but significant hybridisation effects were observed in germination, mainly in seeds produced on wild plants. Crop hybridisation changed seed germination, the after ripening response and secondary dormancy in the crop direction. Morphological and anatomical traits associated with domestication strongly correlated with the observed differences in seed germination and dormancy in crop–wild sunflower hybrids. The large maternal effects along with the evolutionary divergence in seed traits were responsible for the large phenotypic differences observed in crop–wild hybrids with the same genetic composition. Wild‐like seed traits of hybrids suggest that there are no barriers to crop gene introgression at the seed level whereas crop‐like seed traits could be strongly selected against, conditioning the selection of traits expressed later in the life cycle and in the next generations. 相似文献
968.
969.
970.
华北地区莴苣属(广义)植物花粉特征的研究 总被引:2,自引:0,他引:2
本文研究了华北地区莴苣属(广义)7种植物的花粉形态与微形态特征。结果表明:莴苣属(广义)内这些种具有相同的花粉类型。该属的花粉为球形,3孔沟;极面观为三裂圆形,近六边形;赤道面观为圆形;花粉表面具网状纹饰,同胞15个,网脊上有许多小穴和规则排列的刺。与邻近属形成明显差异,使该属成为一个自然类群。新属毛鳞菊属ChaetoserisShihgen.nov.以其花粉网状纹饰的网脊上具不规则排列的短柱而有别于莴苣属(广义)。花粉的形态特征与微形态特征可为菊科属级水平分类提供重要依据。 相似文献