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61.
Of at least 350 species so far recorded within the boundaries of the Niger Inundation Zone, no less than 108 (31%) are wholly or partially of Palaearctic origin. Five main habitats are recognized in the region: wetland, hygrophilous grassland, transition zone, non-flooded areas and aerial. The habitats, the Palaearctic migrants and their possible Ethiopian competitors are described and discussed. The annual Niger flood regime enables Palaearctic waterbirds to find suitable habitats somewhere within the region during all seasons, but most widely during the autumn and winter months of the flood recession. Non-aquatic species inhabiting flood plain grassland are scarce during the Palaearctic autumn, when the growth of vegetation reaches its maximum, becoming commoner and more diverse during the winter months. Wetland warblers of the genera Acrocephalus and Locustella have not been recorded on autumn passage. It is suggested that in some years at least, these and other trans-Saharan migrants from the West Palaearctic overfly the Sahelian latitudes of mid-West Africa. Species inhabiting the transition zone (which increases in area during years of below average rains and floods) are most in evidence at the end of the winter period and during the spring hot dry season, prior to northward trans-Saharan migration. Habitats and species encountered in the non-flooded areas are similar to those recorded in the Sahel zone elsewhere in West Africa. Aerial habitat is utilized by Common Swifts arriving en masse in early August, at the maximum development of the south-westerly monsoon airstream. Several aquatic species and the first few trans-Saharan migrants also occur during the summer rainy season. Some species (e. g. Whiskered Tern, Lesser Kestrel, Turtle Dove, Short-toed Lark and Sand Martin) are most numerous during the late winter and spring hot seasons, when conditions provided by the Inundation Zone may be more suitable for pre-migration feeding than in other parts of the Sahel zone. As yet there is virtually no information available to determine any effects that drought seasons or flood variation may have on the migrant populations. 相似文献
62.
PETER L. BRITTON 《Ibis》1978,120(4):450-466
The commoner birds of a western Kenya papyrus swamp are listed. This avifauna is probably as rich as in any papyrus habitat, dominated by warblers (Sylviinae) in which interspecific competition between species of similar size is reduced by preference for swamp-edge rather than interior habitats, preference for papyrus over standing water, or vertical habitat partitioning. Comparisons with other west Kenyan habitats show that it is a specialized avifauna with low species diversity indices and few species. Density figures for common species are higher than in forest undergrowth or thicket. There is little ecological diversity of habitat in uniform stands of papyrus which form ecological islands, and this high density may be interpreted as a density compensation on a speciespoor ‘island’ compared to a species-rich ‘mainland’. Rainfall is the most seasonally variable climatic factor in west Kenyan papyrus habitats. In this climatic area of Uganda and western Kenya there is a modal egg-laying period from March to June coinciding with the long rains, and a postnuptial primary moult period from June to November, which extends into the short rains of October and November. On average, a complete moult of the remiges takes six or seven months. In general the moult of immatures begins and ends about two months later than adult moult, but birds reared during the short rains had primary moult only half complete at the beginning of the following long rains. There is very little evidence of interrupted moult. The 1969 rainfall pattern was exceptional, with the heaviest rainfall in February, and there is evidence of a correspondingly early onset of postnuptial moult in that year. Females moulted significantly earlier than males in 1969 whereas males moulted marginally earlier in other years, suggesting that female moult schedules are more flexible. 相似文献
63.
Robust density estimation using distance methods 总被引:2,自引:0,他引:2
64.
T. PETER CRIMES 《Lethaia: An International Journal of Palaeontology and Stratigraphy》1975,8(1):35-48
The production and preservation of trilobite resting and furrowing traces is discussed with special reference to specimens from Upper Cambrian strata in North Walcs, Spain, and Poland. Resting excavations are attributed largely or completely to animal activity, and do not normally rcsult from currcnt scouring. Resting excavations, furrows, and many other trace fossils are formed at the sediment-water interface and not bencath a thin sand layer or by burrowing between beds. The factors which allow the initial production of the traces on the ocean floor muds (e.g. cohesion, shear strength, low pore water pressures) will also favour their preservation during deposition of the casting sand. 相似文献
65.
TURPEN JAMES B.; PETER VOLPE E.; COHEN NICHOLAS 《Integrative and comparative biology》1975,15(1):51-61
In the leopard frog (Rana pipiens), thymic lymphocytes do notoriginate from blood-borne stem cells that migrate into thethymus anlage; rather they arise in situ from elements in thethymic rudiment itself. After thymic differentiation, the lymphocytes(or their descendants) leave the thymus and extensively seedthe peripheral lymphoid organs. Indeed, virtually all the lymphocytesin the spleen, kidney, and bone marrow are ontogenically derivedfrom thymic cells. In postmetamorphic life, the thymus representsan organ in which lymphopoiesis is genuinely self-sustaining.Throughout the juvenile life of the frog, there is no indicationof an inward afferent stream of cells entering the intact thymus. 相似文献
66.
HANS‐PETER SCHULTZE DIRK FUCHS SAMUEL GIERSCH CHRISTINA IFRIM WOLFGANG STINNESBECK 《Palaeontology》2010,53(3):689-694
Abstract: The supposed vestige of a cephalopod gladius from Turonian platy limestones at Vallecillo, north‐east Mexico, named Palaeoctopus pelagicus by Fuchs et al. in 2008, is reinterpreted and shown to be a gular plate of a coelacanth fish, possibly of the genus Megacoelacanthus. In addition to the gular plate, two extrascapulars and fin rays of all fins are preserved on one slab and its counterpart. This is the first record of a coelacanth from these lower Turonian strata at Vallecillo, which are rich in fish. 相似文献
67.
68.
Patterns of photobiont diversity were examined in some Nostoc -containing lichens using the nucleotide sequence of the cyanobacterial tRNALeu (UAA) intron. Lichen specimens collected in northwestern USA were analysed and the sequence data were compared with tRNALeu (UAA) intron sequences previously obtained from lichens in northern Europe. Generally, it is the species identity of a lichen rather than the geographical origin of the specimen that determines the identity of the cyanobiont. Identical intron sequences were found in Peltigera membranacea specimens collected in Oregon (USA) and in Sweden, and very similar sequences were also found in Nephroma resupinatum thalli collected in Oregon and Finland. Furthermore, in mixed assemblages where two Peltigera species grew in physical contact with each other, the different lichen species housed different photobiont strains. There is however not a one-to-one relation between mycobiont and photobiont as some intron sequences were found in more than one lichen species, and different intron sequences were found in different samples of some lichen taxa. Peltigera venosa exhibited a higher level of photobiont diversity than any other lichen species studied, and several intron sequences could for the first time be obtained from a single thallus. It is not clear whether this is evidence of lower cyanobiont specificity, or reflects an ability to exhibit different degrees of lichenization with different Nostoc strains. In one specimen of P. venosa , which contained bipartite cyanosymbiodemes and tripartite, cephalodiate thalli, both thallus types contained the same intron sequence. 相似文献
69.
The structure and development of the reproductive organs andcarposporophyte are described for two British species of Gelidium,G. latifolium (Grev.) Born. & Thur. and G. pulchellum (Turn.)Kütz. The gonimoblast does not develop directly from theunchanged carpogonium, as was thought by Kylin, but a swollenmultinucleate cell of irregular outline is formed first, eitherfrom the carpogonium alone, or by the fusion of the carpogoniumand certain neighbouring cells; the gonimoblast develops fromlong, non-septate processes of this multi-nucleate cell. Thesignificance of the fusion of the carpogonium and certain vegetativecells is discussed in relation to the various definitions ofthe auxiliary cell which have been proposed. It is concludedthat an auxiliary cell does not occur in Gelidium. 相似文献
70.