全文获取类型
收费全文 | 549195篇 |
免费 | 49465篇 |
国内免费 | 1001篇 |
出版年
2018年 | 17110篇 |
2017年 | 16064篇 |
2016年 | 13800篇 |
2015年 | 8280篇 |
2014年 | 9058篇 |
2013年 | 12961篇 |
2012年 | 19590篇 |
2011年 | 29530篇 |
2010年 | 23481篇 |
2009年 | 19250篇 |
2008年 | 24663篇 |
2007年 | 27001篇 |
2006年 | 13205篇 |
2005年 | 13586篇 |
2004年 | 13344篇 |
2003年 | 13056篇 |
2002年 | 12520篇 |
2001年 | 19622篇 |
2000年 | 19668篇 |
1999年 | 15913篇 |
1998年 | 5899篇 |
1997年 | 6030篇 |
1996年 | 5832篇 |
1995年 | 5287篇 |
1994年 | 5373篇 |
1993年 | 5272篇 |
1992年 | 13199篇 |
1991年 | 12600篇 |
1990年 | 12489篇 |
1989年 | 12461篇 |
1988年 | 11329篇 |
1987年 | 10774篇 |
1986年 | 9947篇 |
1985年 | 10017篇 |
1984年 | 8193篇 |
1983年 | 7122篇 |
1982年 | 5450篇 |
1981年 | 4813篇 |
1980年 | 4667篇 |
1979年 | 7752篇 |
1978年 | 6027篇 |
1977年 | 5374篇 |
1976年 | 5187篇 |
1975年 | 5595篇 |
1974年 | 5848篇 |
1973年 | 5756篇 |
1972年 | 5820篇 |
1971年 | 5191篇 |
1970年 | 4073篇 |
1969年 | 3763篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Yu. N. Litvinov N. T. Erzhanov N. V. Lopatina T. Zh. Abylkhasanov 《Contemporary Problems of Ecology》2010,3(5):593-596
Small mammals were studied in the Kazakh Uplands in the spring and fall of 2008. The trapping studies revealed 10 species.
Abundances of the animals were low in the four main distinct characteristic biotopes of Bayanaul National Park, but those
of biotope dominants were high. In the Kazakh Uplands, rodents and insectivores are clearly restricted to certain biotopes.
Biodiversity indices for small mammal communities are low, indicating that the community structure is disturbed. 相似文献
993.
An experiment on acceleration and retardation of ontogeny with thyroid manipulation has revealed direct changes in definitive
dentition of pharyngeal bones in Abramis brama bream. As development rate accelerates, the number of teeth reduces to the formula 5-4. When development rate is retarded,
this number increases to the formula 6-5. Moreover, an additional minor row of teeth (1.6–5.1, 2.6–5.2) is formed. The observed
changes in tooth numbers exceed the known variability in natural populations of bream. It is assumed that heterochronies lead
to the changes in the number of teeth. 相似文献
994.
995.
996.
Turtles are among the most intriguing amniotes but their communication and signaling have rarely been studied. Traditionally, they have been seen as basically just silent armored ‘walking stones’ with complex physiology but no altruism, maternal care, or aesthetic perception. Recently, however, we have witnessed a radical change in the perception of turtle behavioral and cognitive skills. In our study, we start by reviewing some recent findings pertaining to various highly developed behavioral and cognitive patterns with special emphasis on turtles. Then we focus on freshwater turtles and use data about their sexual behavior and size sexual dimorphism (SSD) to test whether conspicuous coloration of the head is in these animals related to sexual processes. We found that absence of aggressive mating behavior is statistically associated with the presence of conspicuous coloration on turtles’ heads. It also seems that while species with female-biased SSD are characterised by conspicuously colored head ornaments, in species with male-biased SSD conspicuous coloration is absent. Unlike large females, males thus seem to be under pressure to develop conspicuous coloration and engage in non-aggressive behavior using signaling to succeed in courtship. And finally, we discuss possible roles of head color patterns in turtle communication during mating. 相似文献
997.
998.
C Chatelet J Gaillard Y Pétillot M Louwagie J Meyer 《Biochemical and biophysical research communications》1999,261(3):885-889
Overexpression in Escherichia coli of the fdx4 gene from Aquifex aeolicus has allowed isolation and characterization of the first hyperthermophilic [2Fe-2S](Scys)(4) protein, a homodimer of M = 2 x 12.4 kDa with one [2Fe-2S] cluster per subunit. This protein is undamaged by heating to 100 degrees C for at least three hours. The primary structure, in particular the characteristic distribution of the four cysteine ligands of the metal site, and the spectroscopic properties of the A. aeolicus protein relate it to well characterized [2Fe-2S] proteins from Clostridium pasteurianum and Azotobacter vinelandii. These proteins are also homologous to subunits or domains of hydrogenases and NADH-ubiquinone oxidoreductase (Complex I) of respiratory chains. The A. aeolicus [2Fe-2S] protein is thus representative of a presumably novel protein fold involved in a variety of functions in very diverse cellular backgrounds. 相似文献
999.
The development of spinal cord supports (bony thickenings which extend into the vertebral canal of vertebrae) in primitive (Salamandrella keyserlingii) and derived (Lissotriton vulgaris) salamanders were described. The spinal cord supports develop as the protuberances of periostal bone of the neural arches in the anteroproximal part of the septal collagenous fibers which connect a transverse myoseptum with the notochord and spinal cord, in the septal bundle inside the vertebral canal. Spinal cord supports were also found in some teleostean (Salmo salar, Oncorhynchus mykiss) and dipnoan (Protopterus sp.) fishes. The absence of the spinal cord supports in vertebrates with cartilaginous vertebrae (lampreys, chondrichthyan, and chondrostean fishes) corresponds to the fact that the spinal cord supports are bone structures. The absence of the spinal cord supports in frogs correlates with the lack of the well developed septal bundles inside the vertebral canal. The spinal cord supports are, presumably, a synapomorphic character for salamanders which originated independently of those observed in teleostean and dipnoan fishes. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc. 相似文献
1000.