全文获取类型
收费全文 | 10022篇 |
免费 | 2042篇 |
国内免费 | 1449篇 |
出版年
2024年 | 66篇 |
2023年 | 481篇 |
2022年 | 313篇 |
2021年 | 420篇 |
2020年 | 774篇 |
2019年 | 741篇 |
2018年 | 690篇 |
2017年 | 742篇 |
2016年 | 701篇 |
2015年 | 694篇 |
2014年 | 711篇 |
2013年 | 749篇 |
2012年 | 536篇 |
2011年 | 498篇 |
2010年 | 484篇 |
2009年 | 612篇 |
2008年 | 585篇 |
2007年 | 523篇 |
2006年 | 442篇 |
2005年 | 411篇 |
2004年 | 348篇 |
2003年 | 247篇 |
2002年 | 216篇 |
2001年 | 185篇 |
2000年 | 204篇 |
1999年 | 137篇 |
1998年 | 141篇 |
1997年 | 90篇 |
1996年 | 103篇 |
1995年 | 114篇 |
1994年 | 83篇 |
1993年 | 58篇 |
1992年 | 44篇 |
1991年 | 37篇 |
1990年 | 31篇 |
1989年 | 19篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1986年 | 29篇 |
1985年 | 23篇 |
1984年 | 21篇 |
1983年 | 22篇 |
1982年 | 35篇 |
1981年 | 17篇 |
1980年 | 28篇 |
1979年 | 27篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 6篇 |
1958年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Sequence variability of simian immunodeficiency virus in a persistently infected rhesus monkey 总被引:2,自引:0,他引:2
A juvenile rhesus monkey that was inoculated intravenously with molecularly cloned SIVmac239 became persistently infected. A modified polymerase chain reaction (PCR) procedure was used to specifically amplify full-length envelope (env) gene sequences from DNA extracted from peripheral blood mononuclear cells (PBMC), lymph node tissue, and cells infected with recovered virus at 69 and 93 weeks post-infection. Extensive sequence variability accumulated in vivo in spite of infection with molecularly cloned virus. In the central portion of env. sequence variability was largely confined to three discrete regions. 相似文献
62.
JONAS SANDSTRÖM 《Ecological Entomology》1996,21(1):56-62
Abstract.
- 1 Temporal changes in host adaptation were followed in a local population of the pea aphid, Acyrthosiphon pisum. Aphid clones were collected in one alfalfa and one clover field at three different times. In the spring, first-generation females were collected. Later, in the autumn, females belonging to the last parthenogenetic generation were collected. Lastly, sexual females were collected after mating in autumn and allowed to produce eggs which were hatched. The performance was evaluated on alfalfa and clover. The spring-collected individuals were also assessed on peas.
- 2 On the overwintering hosts clover and alfalfa, the clones performed best on the plant of origin, i.e. negative correlations in performance. Correlations between performance on the temporary summer host, pea, and that on clover/alfalfa were weak or nonsignificant.
- 3 Significant variation in host performance was found within both host fields at spring, which is a prerequisite for changes in clone composition due to selection/migration.
- 4 The clones from alfalfa showed an increase in mean performance on alfalfa between spring and autumn, whereas no changes among the clones from the clover field were observed. This difference in seasonal response between the two fields could have been the result of larger variation in performance among the alfalfa clones and/or a differential tendency to migrate among clones in both fields.
- 5 After sexual recombination in the autumn, mean performance in the alfalfa field returned to the spring level, probably as a result of emergence of new genetic combinations. In the clover field, mean performance did not change significantly over time.
63.
J. BUTTERFIELD 《Ecological Entomology》1996,21(1):9-16
Abstract.
- 1 There was little overlap in the species composition of carabid beetle assemblages sampled below 450 m and above 800 m on an altitude transect and the first axis of a DECORANA ordination was closely correlated with altitude (r11= 0.93, P < 0.001), probably reflecting the differing temperature requirements of different species.
- 2 Life-cycle strategies of two low-altitude species, found predominantly below 600 m, and two species caught above 600 m a.s.l., have been determined, using mandible wear to identify whether the females breed in the calendar year that they emerge as adults or in the year following.
- 3 Nebria salina has an annual cycle at 630 m. Pterostichus madidus and Calathus fuscipes were both biennial at altitudes above 300 m although predominantly annual at low altitude. N.gyllenhali was biennial above 600 m and it is not known whether it is able to switch to an annual cycle at low altitude.
- 4 Both N.salina and N.gyllenhali ceased activity soon after emergence, an adaptation which preserves their mandibles from wear.
- 5 At altitudes of 305 m and 430 m, P.madidus and C.fuscipes entered the breeding season with 33% and 56% reduction in mandible tip length, possibly reducing their reproductive output.
- 6 The necessity for relatively sharp mandibles on entry into the breeding season may restrict the capacity of carabids to respond to a temperature change by switching from annual to biennial cycles, and vice versa, adding support to the suggestion that carabids are more likely to respond to climate change by shifting distributions than by physiological adaptation.
64.
C. Sauvain-Dugerdil G. Boëtsch C. Serrano-Sanchez 《International Journal of Anthropology》1996,11(2-4):3-11
this first paper introduces the topic of the volume, outcomes of a symposium organised at the 13th ICAES held in Mexico in
August 1993. It briefly reminds of the convergence between anthropology and demography up to date and stresses on the need
of real transdisciplinary work in this challenging domain. The aim of the symposium was to make a contribution on the central
theme chosen by the congress — the cultural and biological dimensions of global change — while examining the place of demographic
anthropology in the study of change. The papers presented at the symposium have been organised in three parts which form the
present volume: the composition of population, the choice of spouse and mobility, the reproduction and dynamics of populations.
The basic mechanisms of change are considered through examples at the level of local populations. This also leads to question
the definitions of human groups and to make a “declaration” stressing on the importance of individual heterogeneity and the
arbitrariness and reductive nature of any grouping of individuals, stating therefore the misapprehension of the most recent
scientific work inherent to the rationale of programs of “ethnic cleasing”.
Translated from the french by prof. Derek F. Roberts 相似文献
65.
Human evolution began in East Africa four million years ago, with a transition from an arboreal state to a more terrestrial
one. This evolution seems to be correlated with a large environmental change in East Africa around 2.5 m.y. due to a major
climatic change leading to drier and cooler conditions.
Cenogram analysis (a graphical representation of community structure) can be used to reconstruct the vegetation cover at a
regional scale, and to infer the changing climatic conditions. Using cenogram sequences of different sites along the Rift
Valley, we were able to determine the regional ecological context in which mammals and hominids have evolved in East Africa
during the last 3 million years.
Between 3.5 and 2 m.y., during a general climatic change, successive faunas of South Tanzania reflect the progressive opening
of their environment. In contrast around Lake Turkana a mosaic of isolated dry and wet habitats were present throughout this
period. At this time, the Rift seems to have been spatially structured in several basins isolated from one other, and isolated
faunas experienced separate speciation events (particularly with the appearance ofHomo genus).
After 2 m.y., the disappearance of the isolating barriers on one hand, and a regional increase in aridity, on the other hand,
led to more homogenous faunas arising throughout the region. Replacements of mammal species occurred (especiallyHomo erectus replacingHomo habilis) and several others mammal species, including australopithecines, disappeared during this same period. 相似文献
66.
J. D. Paterson 《International journal of primatology》1996,17(4):585-611
I analyzed somatometric measurements from subsets of the Texas and Oregon transplanted troops of Japanese macaques(Macaca fuscata) to reveal secular changes in body size and shape. Body weights of the Texas population (N = 59) are lower than those of the
Oregon population(N = 49) and the founding population from Arashiyama. The adult weights of the Oregon population are significantly higher than the
founding population from Mihara. There are significant differences in adult circumferential measures and in skinfolds, which
are correlated with the increased weight of the Oregon macaques. The adult Texas macaques have longer limb segments in comparison
with the adult Oregon troop members, while the latter have significantly longer heads and trunks. Examination of the developing
morphological trends through regression analyses on the complete sample suggests distinctive growth patterns for each population.
Members of the Texas population start with smaller initial measurements but hold a steeper growth pattern for limb segments,
while the Oregon macaques start larger in most measures and show lower growth rates. I argue that these differences in both
somatometry and growth patterns are related to the differing climatic conditions under which the translocated macaques have
lived. This set of analyses supports the basic arguments for Bergmann’s rule and Allen’s rule. 相似文献
67.
Leaf tolerance to high temperatures, as determined by electrolyte leakage and chlorophyll a fluorescence, was compared for Artemisia tridentata (Asteraceae), a widespread shrub of the Great Basin, Colorado Plateau, and western slope of the Rocky Mountains, and Potentilla gracilis (Rosaceae), a herbaceous forb common to high-elevation meadows of the western United States. Species-specific and treatment-specific differences in leaf temperature, high-temperature tolerance and chlorophyll a fluorescence from photosystem II were compared, to test the hypothesis that plants at ecosystem borders will exhibit species-specific responses to climate change. Measurements were made for plants exposed to a climate change warming manipulation on a major ecosystem border at the Rocky Mountain Biological Laboratory, Colorado, United States, in July and August 1995. In July, daily maximal leaf temperatures were significantly higher for P. gracilis than for A. tridentata. Leaf temperatures were slightly lower in August than July for leaves of both species, on control and heated plots, despite the fact that daily maximum air temperatures were not significantly different for the two months. High-temperature tolerance was determined for leaves treated for 1 h at temperatures ranging from 15°C to 65°C. LT50 was approximately 46°C for both species on control plots, but was 43°C for leaves of both species from heated plots, contrary to the predictions of the hypothesis. No shift in LT50 (acclimation) was apparent between July and August. Changes in chlorophyll a fluorescence from photosystem II (F
V
/F
M
) were used to characterize the photosynthetic response to high temperatures. For both A. tridentata and P. gracilis in July, F
V
/F
M
was about 0.7, but decreased for temperatures above 40°C. The results suggest that plant responses to global warming at ecosystem borders may be influenced by factors other than leaf-level physiological tolerance to elevated temperatures. 相似文献
68.
S. Zielinski H. O. Pörtner 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1996,166(8):492-500
The intertidal wormSipunculus nudus was exposed to various temperatures for an analysis of the integrated changes in energy and acid-base status. Animals were
incubated in sea water or maintained for up to 8 days at 4 and 0°C while dwelling in the sediment. Cannulation of the animals
prior to experimentation allowed the analysis of blood gas parameters (
,
and pH).
fell to 0 torr within 8 days at 0°C. A simultaneous reduction of ventilatory activity was derived from measurements of the
pattern of coelomic fluid pressure changes associated with ventilatory movements. The increase in
and an onset of anaerobic metabolism, indicated by the accumulation of end products like acetate and propionate both in the
coelomic fluid and the body wall musculature, led to the development of a progressive acidosis and a deviation from the alphastat
regulation of intracellular pH seen in unburied animals. The drop in intracellular pH together with the depletion of the adenylates
and the phosphagen, phospho-l-arginine, reflect a significant decrease in the Gibb's free-energy change of ATP hydrolysis. These changes are interpreted
to indicate lethal cold injuries, because recovery was not possible when the animals were returned to 12°C after more than
2 days of exposure to 0°C. A low critical temperature indicating the onset of cold-induced anaerobiosis is concluded to exist
below 4°C owing to the insufficient response of the ventilatory system to the developing hypoxia. 相似文献
69.
REYNALDO LUIZ VICTORIA FERNANDO FERNANDES† LUIZ ANTONIO MARTINELLI MARISA DE CÁSSIA PICCOLO PLINIO BARBOSA DE CAMARGO SUSAN TRUMBORE‡ 《Global Change Biology》1995,1(3):165-171
Measurements of the organic carbon inventory, its stable isotopic composition and radiocarbon content were used to deduce vegetation history from two soil profiles in arboreal and grassy savanna ecotones in the Brazilian Pantanal. The Pantanal is a large floodplain area with grass-dominated lowlands subject to seasonal flooding, and arboreal savanna uplands which are only rarely flooded. Organic carbon inventories were lower in the grassy savanna site than in the upland arboreal savanna site, with carbon decreasing exponentially with depth from the surface in both profiles. Changes in 13C of soil organic matter (SOM) with depth differed markedly between the two sites. Differences in surface SOM 13C values reflect the change from C3 to C4 plants between the sites, as confirmed by measurements of 13C of vegetation and the soil surface along a transect between the upland closed-canopy forest and lowland grassy savanna. Changes of 13C in SOM with depth at both sites are larger than the 3–4 per mil increases expected from fractionation associated with organic matter decomposition. We interpret these as recording past changes in the relative abundance of C3 and C4 plants at these sites. Mass balances with 14C and 13C suggest that past vegetational changes from C3 to C4 plants in the grassy savanna, and in the deeper part of the arboreal savanna, occurred between 4600 and 11 400 BP, when major climatic changes were also observed in several places of the South American Continent. The change from C4 to C3, observed only in the upper part of the arboreal savanna, was much more recent (1400 BP), and was probably caused by a local change in the flooding regime. 相似文献
70.
Gerd Esser 《Plant Ecology》1995,121(1-2):175-188
The High Resolution Biosphere Model (HRBM), which has been developed by the group of the author, was used to investigate the carbon balance of the vegetation and the soil in the ecosystems of Monsoon Asia in comparison to the rest of the world. The HRBM is a global grid-based (0.5 degree resolution) model with a monthly time step. It includes modules for natural vegetation, land use, vegetation fires, vegetation composition. A historical carbon budget was calculated for the period 1860–1978 and, on a global scale, validated using atmospheric CO2 data. Based on the per-country development of the population and their requirements, different reasonable scenarios were used to investigate the potential impacts of land use and deforestation in the period 1990–2050. The HRBM calculates considerable contributions of Monsoon Asia to the global CO2 emissions due to land use changes in the past. Between 1860 and 1978, about 1/4 of the global releases from land use changes came from South Asian and Southeast Asian biota. The future contributions in the period 1990–2050 depend on the assumed development of the agricultural methods. If the intensity of agriculture and the agricultural productivity will stay the same as in the 1980s, there will be a strong need to increase agricultural areas, and thus deforestation will dominate. If there will be a change over to intensive methods of agricultural production, the presently used areas might be sufficient to provide resources to the growing population. 相似文献