全文获取类型
收费全文 | 1364篇 |
免费 | 188篇 |
国内免费 | 201篇 |
出版年
2024年 | 4篇 |
2023年 | 27篇 |
2022年 | 29篇 |
2021年 | 46篇 |
2020年 | 78篇 |
2019年 | 68篇 |
2018年 | 57篇 |
2017年 | 63篇 |
2016年 | 65篇 |
2015年 | 66篇 |
2014年 | 61篇 |
2013年 | 84篇 |
2012年 | 75篇 |
2011年 | 87篇 |
2010年 | 64篇 |
2009年 | 76篇 |
2008年 | 56篇 |
2007年 | 62篇 |
2006年 | 59篇 |
2005年 | 50篇 |
2004年 | 59篇 |
2003年 | 46篇 |
2002年 | 49篇 |
2001年 | 52篇 |
2000年 | 46篇 |
1999年 | 44篇 |
1998年 | 33篇 |
1997年 | 38篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 13篇 |
1992年 | 14篇 |
1991年 | 19篇 |
1990年 | 15篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 9篇 |
1984年 | 4篇 |
1983年 | 7篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1974年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有1753条查询结果,搜索用时 281 毫秒
1.
The environmental conditions to which juvenile barnacle geese (Branta leucopsis) were exposed during growth were found to affect their body size at fledging as well as their final adult body size. Small juveniles showed compensatory growth from the time of fledging up to one year of age, but this did not fully compensate the differences in body size that were established before fledging. The variation in protein content in plants eaten during growth could probably explain the observed body size differences, sometimes of more than 10%, between different categories of adult geese. Our results imply that one cannot infer selection on morphological characters from differences between samples of adult birds from different localities or from different cohorts within a population, without first showing that environmental conditions during growth do not affect the development of the characters under study. 相似文献
2.
Arvicanthis is an African murid, found throughout sub-Saharan Africa, Sudan and Egypt. Although in the past 10 years several studies have been carried out to assess its systematics, there is still a need for a general revision of the genus. In this study the morphometric relationships between 71 populations throughout the range were investigated. A three-dimensional geometric morphometric approach was used to assess differences in the size and shape of the skull. These were related to the different biogeographical domains characterizing the range of the genus and to molecular and karyotypic phylogenies. Results agree only in part with phylogeny, and show a close relationship with the environmental backgrounds of each species. It is therefore suggested that the adaptation of Arvicanthis to local environment has played an important role in the phenotypic evolution of the skull. This leads to problems in taxonomic definitions based on morphometrics, which should not be used without comparison with other independently derived characters such as the DNA and the karyotype. 相似文献
3.
JACQUES Destombes ET JEAN-LOUIS Henry 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1987,20(2):129-139
A re-examination of the trilobite Baniaspis globosa Destombes (Phacopina) from the Ashgill of the Anti-Atlas (Morocco) shows that it has several derived characters which allow us to classify it in the family Calmoniidae. Calmoniids were characteristic members of the Malvino-Kaffric province during the Lower Devonian. Hence we regard the origins of this palaeogeographic province as located on the northern Gondwanan margin as early as Ashgill, or even Caradoc, times. We agree with those authors who have suggested that the trilobite associations present in the Anti-Atlas, the Montagne Noire (France) and Bohemia during the earlier Ordovician argue against the existence of a 'proto-Tethys' ocean. 相似文献
4.
Patterns of variation in tail ornament size in birds 总被引:5,自引:0,他引:5
RAUNO V. ALATALO JACOB HÖGLUND ARNE LUNDBERG 《Biological journal of the Linnean Society. Linnean Society of London》1988,34(4):363-374
In recent years several different kinds of sexual selection models have been developed, and tail ornaments in birds have frequently been used as an example of a sexually selected character where the models might apply. However, very little is known about intra- and interpopulation variation in ornament size. We have studied the elongated tail ornaments in four species of whydahs Vidua , the forktailed flycatcher Tyrannus savana and the Asian paradise flycatcher Terpsiphone paradisi. Ornaments were relatively longer in males with the longest tarsi ('heterogony' with positive allometry). Also, tail lengths were remarkably variable within each geographical area, the coefficient of variation (average = 11%) being three times as high as for body size characters. Models, with female preference of ornaments bearing no relation to male viability, usually generate lines of neutral equilibria. Thus, they predict extraordinary variation in ornaments between populations. However, elongated tail ornaments did not show higher geographical variation than the body size characters, suggesting that there is no line of equilibria for these ornaments. 相似文献
5.
6.
辽宁省啮齿动物的地理区划 总被引:3,自引:0,他引:3
辽宁省的啮齿动物,过去曾在局部地区进行过调查(伍律等,1958;董谦等,1962;林兰泉等,1965),我站自1961年以来对全省作了较为全面的调查工作。关于啮齿动物的地理区划问题,肖增祜等(1978)曾作过区划拟议,将辽宁省初步划分为三个动物地理省(辽东山地丘陵省、辽西山地丘陵省、辽河平原省)。本文根据1958—1981年全省啮齿动物的调查资料,将辽宁省划分为五个啮齿动物地理省,现作如下叙述和讨论。 相似文献
7.
喀左县农业生态系统能流的研究 总被引:4,自引:0,他引:4
本文根据辽宁省西部一个典型具(喀左县)农业生态系统的调查,分析该系统八十年代初期基本特征,进而提出改善该农业生态系统的措施。 整个系统由七个组分(或亚系统)组成(农田、牲畜、果园、人工林、人工草地、天然灌丛草地、当地居民)。作者根据有关统计数据和调查资料估算出该系统1981—1983年期间年平均流量,其中包括流入和流出能量以及系统内各组分之间的能流量。最后讨论了调整畜牧业结构,提高工业能量、生物质能量利用率和林分改造等问题。 相似文献
8.
杯叶科是食鱼的爬行类、鸟类、哺乳类寄生的吸虫。本文修订了以往在福建报道的虫种,叙述了一个新属三个新种,取消了前此所定的一个属及种[Tangiella parovipara(Faust and Tang,1938)],认为是存疑的种。本文还报道了盖前冠吸虫 Prosostephanus Tubangu,1922的终末宿主河獭,从而作者(Tang,1941)完成了其生活史的研究。本文比较了从河獭所得的标本与从描、犬所获标本各器官的测量数据,发现由最佳的终末宿住所得标本,其个体及各器官和卵子的测量数据均较大。 相似文献
9.
云南景颇族的体质特征 总被引:17,自引:10,他引:7
本文调查了云南景颇族261人(男105人,女156人)。年龄为成年人(男24-60岁,女23-55岁)。主要测量均值与现代中国汉族和云南省各少数民族比较和聚类分析的结果表明,景颇族属黄种人的东亚类型,但也有南亚类型特征表现。体质特征与傣族、哈尼族和彝族接近,与基诺族和布郎族较远。 相似文献
10.
湘南山区农业生态系统能量分析 总被引:3,自引:0,他引:3
一、引言由于农业系统的复杂性和存在较多的随机因素,对农业系统实行全面定量研究是比较困难的,仅从定性的描述和一些经济指标来判定系统的优劣及系统的特征是不理想的。而从能量观点对农业生态系统加以分析则是一个全面且能定量的方法,并且具有较强的客观性,能够反映农业生态系统最基本、最本质的关系。 相似文献