首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   474篇
  免费   46篇
  国内免费   28篇
  2024年   2篇
  2023年   6篇
  2022年   15篇
  2021年   18篇
  2020年   12篇
  2019年   24篇
  2018年   23篇
  2017年   18篇
  2016年   16篇
  2015年   10篇
  2014年   29篇
  2013年   22篇
  2012年   19篇
  2011年   21篇
  2010年   14篇
  2009年   22篇
  2008年   24篇
  2007年   34篇
  2006年   17篇
  2005年   12篇
  2004年   17篇
  2003年   26篇
  2002年   27篇
  2001年   17篇
  2000年   10篇
  1999年   7篇
  1998年   14篇
  1997年   2篇
  1996年   10篇
  1995年   5篇
  1994年   4篇
  1993年   5篇
  1992年   5篇
  1991年   7篇
  1990年   4篇
  1989年   1篇
  1988年   4篇
  1987年   5篇
  1986年   1篇
  1985年   4篇
  1984年   3篇
  1983年   1篇
  1982年   2篇
  1981年   1篇
  1980年   3篇
  1979年   3篇
  1978年   1篇
  1976年   1篇
排序方式: 共有548条查询结果,搜索用时 984 毫秒
541.
The question of whether the modern evolutionary synthesis requires an extension has recently become a topic of discussion, and a source of controversy. We suggest that this debate is, for the most part, not about the modern synthesis at all. Rather, it is about the extent to which genetic mechanisms can be regarded as the primary determinants of phenotypic characters. The modern synthesis has been associated with the idea that phenotypes are the result of gene products, while supporters of the extended synthesis have suggested that environmental factors, along with processes such as epigenetic inheritance, and niche construction play an important role in character formation. We argue that the methodology of the modern evolutionary synthesis has been enormously successful, but does not provide an accurate characterization of the origin of phenotypes. For its part, the extended synthesis has yet to be transformed into a testable theory, and accordingly, has yielded few results. We conclude by suggesting that the origin of phenotypes can only be understood by integrating findings from all levels of the organismal hierarchy. In most cases, parts and processes from a single level fail to accurately explain the presence of a given phenotypic trait.  相似文献   
542.
The activity of six digestive enzymes (amylase, chitinase, trypsin, chymotrypsin, carboxypeptidase A, pepsin) was examined in the water-soluble contents of the pellets egested by ten species of raptors (kestrel, saker, lanner, goshawk, barn owl, tawny owl, little owl, long-eared owl, African great-owl and steppe eagle). All the enzymes studied were present in the pellets from these birds, except for chitinase which was not detected in the pellets of the goshawk and the steppe eagle, and amylase and carboxypeptidase absent in the material egested by the lanner. The origin of the enzymes studied was examined. Pancreatic enzymes, which are present in the pellets, arise from a reflux of intestinal fluid into the stomach. The importance of this phenomenon is discussed. The acidity of the pellets was measured. Relations existing between the type of food, characteristics of the pellet and the digestive process in raptors are analysed. The evolutionary advantage of pellet egestion is discussed.  相似文献   
543.
Case studies have a long tradition in biomedical research. Here, I will analyze an important person from a chronobiological aspect, Santa Claus. Although it might be dangerous for researchers to publish analyses about Santa Claus, because, given the possibility that he may be unsatisfied with my analysis, this could lead to an embargo of parcels for my whole family. Nevertheless, some intrepid researches already diagnosed Santa Claus, ending up in some important research results. A search in Web of Science revealed only N = 224 publications about Santa Claus, which is really low. The question, I am addressing here is, whether Santa Claus is an “evening owl” or a night chronotype. In this report, I summarize the facts known about Santa Claus on the one side and about evening chronotypes on the other and sum up these results in a “vote counting” manner as pros and cons. The results are summarized in Table 1. In total, there is 12 times support for the hypothesis, 2 times against and 1 equivocal (Table 1). This result is significant (binomial test, p = 0.013). Therefore, the conclusion is “Santa Claus is an evening type”.  相似文献   
544.
545.
Pellet forming and fragmentation in liquid culture of Streptomyces griseus   总被引:1,自引:0,他引:1  
Pellets of Streptomyces griseus formed during submerged liquid culture were effectively dispersed in Tris/HCl buffer at 37 °C. However, inclusion of Tris/HCl in the culture medium did not lead to dispersed growth. In order to decrease the pellet formation in batch culture, a large inoculum of spores was required and dispersed growth was further favoured by culture at 37 °C. This method was also used to achieve dispersed culture of S. coelicolor A3(2). Autolytic enzyme activity was presumed to be responsible for fragmentation of pellets.  相似文献   
546.
以学科建设和现代管理理论为指导,通过系统总结加强学科建设的实践经验,提出了“ 人才+设备=技术 , 技术+学术=水平 , 水平+市场=效益 , 效益+和谐=发展 ”的基本思路和“一把钥匙开一把锁”的现代医院学科建设辩证策略,并对上述4个公式和1个公理的内在特点和逻辑关系进行了理论和实证研究,对指导现代医院学科建设具有参考价值。  相似文献   
547.
In a short work called De conceptione appended to the end of his Exercitationes de generatione animalium (1651), William Harvey developed a rather strange analogy. To explain how such marvelous productions as living beings were generated from the rather inauspicious ingredients of animal reproduction, Harvey argued that conception in the womb was like conception in the brain. It was mostly rejected at the time; it now seems a ludicrous theory based upon homonymy. However, this analogy offers insight into the structure and function of analogies in early modern natural philosophy. In this essay I hope to not only describe the complex nature of Harvey’s analogy, but also offer a novel interpretation of his use of analogical reasoning, substantially revising the account offered by Guido Giglioni (1993). I discuss two points of conceptual change and negotiation in connection with Harvey’s analogy, understanding it as both a confrontation between the border of the natural and the supernatural, as well as a moment in the history of psychology. My interpretation touches upon a number of important aspects, including why the analogy was rejected, how Harvey systematically deployed analogies according to his notions of natural philosophical method, how the analogy fits into contemporary discussions of analogies in science, and finally, how the analogy must be seen in the context of changing Renaissance notions of the science of the soul, ultimately confronting the problem of how to understand final causality in Aristotelian science. In connection with the last, I conclude the essay by turning to how Harvey embeds the analogy within a natural theological cosmology.  相似文献   
548.
Aim Fossil pollen spectra from lake sediments on the Tibetan Plateau have been used for qualitative climate reconstruction, but no modern pollen–climate calibration set based on lake sediments is available to infer past climate quantitatively. This study aims to develop such a dataset and apply it to fossil data. Location The Tibetan Plateau, between 30 and 40° N and 87 and 103° E. Methods We collected surface sediments from 112 lakes and analysed them palynologically. The lakes span a wide range of mean annual precipitation (Pann; 31–1022 mm), mean annual temperature (Tann; −6.5 to 1 °C), and mean July temperature (TJuly; 2.6–19.7 °C). Redundancy analysis showed that the modern pollen spectra are characteristic of their respective vegetation types and local climate. Transfer functions for Pann, Tann and TJuly were developed with weighted averaging partial least squares. Model performance was assessed by leave-one-out cross-validation. Results The root mean square errors of prediction (RMSEP) were 104 mm (Pann), 1.18 °C (Tann) and 1.17 °C (TJuly). The RMSEPs, when expressed as percentages of the gradient sampled, were 10.6% (Pann), 15.7% (Tann) and 11.9% (TJuly). These low values indicate the good performance of our models. An application of the models to fossil pollen spectra covering the last c. 50 kyr yielded realistic results for Luanhaizi Lake in the Qilian Mountains on the north-eastern Tibetan Plateau (modern Pann 480 mm; Tann−1 °C). Tann and Pann values similar to present ones were reconstructed for late Marine Isotope Stage 3, with minimum values for the Last Glacial Maximum (c. 300 mm and 2 °C below present), and maximum values for the early Holocene (c. 70 mm and 0.5 °C greater than present). Main conclusions The modern pollen–climate calibration set will potentially be useful for quantitative climate reconstructions from lake-sediment pollen spectra from the Tibetan Plateau, an area of considerable climatic and biogeographical importance.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号