全文获取类型
收费全文 | 62637篇 |
免费 | 5689篇 |
国内免费 | 40篇 |
专业分类
68366篇 |
出版年
2022年 | 391篇 |
2021年 | 746篇 |
2020年 | 470篇 |
2019年 | 574篇 |
2018年 | 770篇 |
2017年 | 706篇 |
2016年 | 1230篇 |
2015年 | 2048篇 |
2014年 | 2365篇 |
2013年 | 3092篇 |
2012年 | 4014篇 |
2011年 | 3993篇 |
2010年 | 2640篇 |
2009年 | 2424篇 |
2008年 | 3471篇 |
2007年 | 3686篇 |
2006年 | 3536篇 |
2005年 | 3298篇 |
2004年 | 3314篇 |
2003年 | 3224篇 |
2002年 | 2999篇 |
2001年 | 644篇 |
2000年 | 439篇 |
1999年 | 718篇 |
1998年 | 883篇 |
1997年 | 638篇 |
1996年 | 585篇 |
1995年 | 571篇 |
1994年 | 545篇 |
1993年 | 594篇 |
1992年 | 544篇 |
1991年 | 459篇 |
1990年 | 450篇 |
1989年 | 443篇 |
1988年 | 442篇 |
1987年 | 436篇 |
1986年 | 439篇 |
1985年 | 499篇 |
1984年 | 611篇 |
1983年 | 555篇 |
1982年 | 671篇 |
1981年 | 724篇 |
1980年 | 677篇 |
1979年 | 413篇 |
1978年 | 460篇 |
1977年 | 449篇 |
1976年 | 390篇 |
1975年 | 345篇 |
1974年 | 406篇 |
1973年 | 352篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Human migration is nonrandom. In small scale societies of the past, and in the modern world, people tend to move to wealthier, safer, and more just societies from poorer, more violent, less just societies. If immigrants are assimilated, such nonrandom migration can increase the occurrence of culturally transmitted beliefs, values, and institutions that cause societies to be attractive to immigrants. Here we describe and analyze a simple model of this process. This model suggests that long run outcomes depend on the relative strength of migration and local adaptation. When local adaption is strong enough to preserve cultural variation among groups, cultural variants that make societies attractive always predominate, but never drive alternative variants to extinction. When migration predominates, outcomes depend both on the relative attractiveness of alternative variants and on the initial sizes of societies that provide and receive immigrants. 相似文献
82.
83.
84.
85.
86.
87.
88.
89.
Clade-specific morphological diversification and adaptive radiation in Hawaiian songbirds. 总被引:3,自引:0,他引:3
Irby J Lovette Eldredge Bermingham Robert E Ricklefs 《Proceedings. Biological sciences / The Royal Society》2002,269(1486):37-42
The Hawaiian honeycreepers are a dramatic example of adaptive radiation but contrast with the four other songbird lineages that successfully colonized the Hawaiian archipelago and failed to undergo similar diversification. To explore the processes that produced the diversity dichotomy in this insular fauna, we compared clade age and morphological diversity between the speciose honeycreepers and the comparatively depauperate Hawaiian thrushes. Mitochondrial-DNA-based genetic distances between these Hawaiian clades and their continental sister taxa indicate that the ancestral thrush colonized the Hawaiian Islands as early as the common ancestor of the honeycreepers. This similar timing of colonization indicates that the marked difference in diversity between the Hawaiian honeycreeper and thrush clades is unlikely to result from differences in these clades' tenures within the archipelago. If time cannot explain the contrasting diversities of these taxa, then an intrinsic, clade-specific trait may have fostered the honeycreeper radiation. As the honeycreepers have diversified most dramatically in morphological characters related to resource utilization, we used principal components analyses of bill characters to compare the magnitudes of morphological variation in the ancestral clades from which the Hawaiian honeycreeper and thrush lineages are derived, the Carduelini and Turdinae respectively. Although the Carduelini share a more recent common ancestor and have a lower species diversity than the Turdinae, these finch-like relatives of the honeycreepers exhibit significantly greater variation in bill morphology than do the continental relatives of the Hawaiian thrushes. The higher magnitude of morphological variation in the non-Hawaiian Carduelini suggests that the honeycreepers fall within a clade exhibiting a generally high evolutionary flexibility in bill morphology. Accordingly, although the magnitude of bill variation among the honeycreepers is similar to that of the entire passerine radiation, this dramatic morphological radiation represents an extreme manifestation of a general clade-specific ability to evolve novel morphologies. 相似文献
90.
Lee D. Major Thomas S. Partridge Joy Gardner Stephen J. Kent Robert de Rose Andreas Suhrbier Wayne A. Schroder 《PloS one》2013,8(2)
SerpinB2, also known as plasminogen activator inhibitor type 2, is a major product of activated monocytes/macrophages and is often strongly induced during infection and inflammation; however, its physiological function remains somewhat elusive. Herein we show that SerpinB2 is induced in peripheral blood mononuclear cells following infection of pigtail macaques with CCR5-utilizing (macrophage-tropic) SIVmac239, but not the rapidly pathogenic CXCR4-utilizing (T cell-tropic) SHIVmn229. To investigate the role of SerpinB2 in lentiviral infections, SerpinB2−/− mice were infected with EcoHIV, a chimeric HIV in which HIV gp120 has been replaced with gp80 from ecotropic murine leukemia virus. EcoHIV infected SerpinB2−/− mice produced significantly lower anti-gag IgG1 antibody titres than infected SerpinB2+/+ mice, and showed slightly delayed clearance of EcoHIV. Analyses of published microarray studies showed significantly higher levels of SerpinB2 mRNA in monocytes from HIV-1 infected patients when compared with uninfected controls, as well as a significant negative correlation between SerpinB2 and T-bet mRNA levels in peripheral blood mononuclear cells. These data illustrate that SerpinB2 can be induced by lentiviral infection in vivo and support the emerging notion that a physiological role of SerpinB2 is modulation of Th1/Th2 responses. 相似文献