全文获取类型
收费全文 | 4729篇 |
免费 | 645篇 |
国内免费 | 397篇 |
出版年
2024年 | 13篇 |
2023年 | 150篇 |
2022年 | 109篇 |
2021年 | 200篇 |
2020年 | 223篇 |
2019年 | 241篇 |
2018年 | 219篇 |
2017年 | 208篇 |
2016年 | 241篇 |
2015年 | 268篇 |
2014年 | 275篇 |
2013年 | 366篇 |
2012年 | 269篇 |
2011年 | 241篇 |
2010年 | 182篇 |
2009年 | 242篇 |
2008年 | 242篇 |
2007年 | 296篇 |
2006年 | 221篇 |
2005年 | 200篇 |
2004年 | 168篇 |
2003年 | 151篇 |
2002年 | 138篇 |
2001年 | 120篇 |
2000年 | 86篇 |
1999年 | 93篇 |
1998年 | 71篇 |
1997年 | 60篇 |
1996年 | 66篇 |
1995年 | 48篇 |
1994年 | 58篇 |
1993年 | 39篇 |
1992年 | 40篇 |
1991年 | 28篇 |
1990年 | 27篇 |
1989年 | 32篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 18篇 |
1985年 | 7篇 |
1984年 | 13篇 |
1983年 | 13篇 |
1982年 | 10篇 |
1981年 | 12篇 |
1980年 | 12篇 |
1979年 | 8篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有5771条查询结果,搜索用时 140 毫秒
31.
The phytoplankton community of the Belarus Lakes Naroch, Myastro and Batorino, which have a Trophic State Index of 42.3, 60.7 and 66.8, respectively, underwent drastic changes to their structure during the period between 1968 and 2012. Thanks to an extensive monitoring program, these changes were well-documented and were qualitatively interpreted as signs of the community destabilization. The main objective of this study was the quantification of the ecological stability of the phytoplankton community in the Naroch Lakes. The approach to the quantification of ecological stability was based on defining the stability index as an inverse of the Euclidean Distance between the current and the reference states of the algal community (EuD-approach). The stability of the phytoplankton community was characterized by two indices: a “combined” index (SI[Comb]), and a “total community” index (SI[TotB]). SI[Comb] was calculated based on the individual taxonomic group biomasses and thus characterizes the stability of a community structure. SI[TotB] was calculated based on the values of the total algal biomass. Analyses of the results of this study extended the plausibility of the EuD-approach for the quantification of lake phytoplankton stability and allowed us to identify the dynamics of the stability of the Naroch Lakes phytoplankton. For the Naroch Lakes, we observed relatively larger SI[TotB] values in comparison with the SI[Comb] values. The results enabled us to examine the relationship between the lake trophic status and the stability of the phytoplankton community. 相似文献
32.
Assessment of the conservation status of partridges and pheasants in South East Asia 总被引:3,自引:2,他引:1
The recent compilation of IUCN Action Plans for partridges and pheasants has highlighted the need for locality data to be used in conservation evaluation. Here we used locality data to measure changes in the proportion of localities from which each of 25 South East Asian species has been recorded, changes in their geographic extent and the amount of habitat that is potentially available to them. These are measures that use the data which are most readily available. Single landmass endemics from both montane and lowland forests appear to have fared badly with peacock pheasants Polyplectron and hill-partridges Arborophila suffering particularly large declines, although for montane forest species, these apparent declines are probably the result of relatively less survey effort in highland areas. Species previously widespread that give cause for concern include the green peafowl, Pavo muticus, and the blue-breasted quail, Coturnix chinensis. Conservation priorities include action on behalf of lowland forest specialists, some of which have suffered substantial reductions in the area of habitat that is potentially available. Surveys are required on the hills and mountains of Borneo, the Malay Peninsula and Sumatra, and in the Lesser Sundas. Increased resolution of vegetation data would permit more accurate prediction of species ranges. 相似文献
33.
34.
35.
Eliab Estrada-Corts Hctor R. Vera-Avila Jorge Urrutia-Morales Eugenio Villagmez-Amezcua Hctor Jimnez-Severiano Csar A. Mejía-Guadarrama M. Teresa Rivera-Lozano Hctor G. Gmez-Vzquez 《Animal reproduction science》2009,116(3-4):282-290
The objective was to determine the effect of body energy stores, evaluated by a body mass index (BMI), and food intake (FI), on the length of the anovulatory period and ovarian activity during the seasonal reproductive transitions in Creole goats. Non-pregnant, non-lactating Creole goats (n = 28) were fed to induce two different BMI conditions: Greater (GBMI; n = 15), and Lesser (LBMI; n = 13). Each BMI group was divided into two sub-groups, which were either feed restricted (FR) or non-feed restricted (NFR). Goats in the NFR groups received a diet containing 100% of the daily maintenance requirements (basal diet), while restricted goats were subjected to alternated periods, receiving 100% (11 d) and 60% (10 d) of the basal diet, during the entire experimental period. The experiment started after does were treated to synchronize time of estrus. Serum progesterone was determined in samples obtained twice a week, and used as a criterion for determining ovulations. During the transition to the anovulatory period three transrectal ovarian ultrasonographic scans were performed in a sub-group of 12 goats (n = 3 for each treatment combination). The diameter of the largest follicle (LFD) and the total number of antral follicles ≥2 mm (TAF) were recorded. Ultrasonographic ovarian scans were performed at 21, 42 and 63 days after the beginning of the experiment, concurrently with the end of each feed restriction period. The variables of response associated with ovulation were not influenced by BMI or BMI × FI interaction. However, FI influenced length of anovulatory season, as the anovulatory period was 30 d longer (P < 0.05) in the FR group as compared with the NFR group. Independently of treatments, TAF and LFD decreased from the first to the third ultrasonographic ovarian scan (13.2, 10.8 and 4.4 follicles; 3.7, 2.7 and 2.3 mm). Nevertheless, in PER 1 the number of TAF was greater (P < 0.05) in the FR as compared with NFR group and the GBMI group had a larger LFD (P < 0.05) as compared to the LBMI group. It is concluded, that temporal restriction in feed intake could affect the time of cessation and initiation of ovulations during the periods of transition to seasonal anestrus and return to estrous activity, and increase the length of the anovulatory period. In addition, ovarian follicular development during transition into the anovulatory period is differentially influenced by food intake and the status of body energy stores. 相似文献
36.
37.
Dominant males of nonseasonally breeding species should have more opportunity than seasonal breeders to monopolize access to estrous females and thus enhance their reproductive success. They may also use other strategies for maximizing reproductive success such as producing offspring by relatively high ranking females. To test these hypotheses, the paternity of 35 (80%) of 44 Macaca fascicularis offspring, born over a 28-month period, was established using electrophoretically and serologically defined genetic markers. The social ranks of each of the seven potential fathers and 26 potential mothers were determined by recording outcomes of dyadio agonistic interactions. No evidence could be found to support either hypothesis. Further, in view of previously reported significant, positive correlations between male social rank and sexual activity rates in this study group, sexual activity rates do not appear to predict the number of offspring a male sires in this species. 相似文献
38.
Daniel Pérusse 《Human nature (Hawthorne, N.Y.)》1994,5(3):255-278
Most research on mate choice in modern societies is based on data that may or may not reflect actual mating behavior (e.g.,
stated preferences, personal advertisements). In the present study, real-life matings were reported by a large representative
sample of men and women (N = 1,133). These data were used to test an evolutionary model in which mate choice is hypothesized to depend on resources
potentially contributed to reproduction by each sex. Consistent with the model, it was found that (a) men (but not women) of higher social status acquire more mating partners, suggesting that male status is an important criterion
in female choice; (b) women’s (but not men’s) number of partners decreases linearly with age, suggesting that female reproductive potential is
an important criterion in male choice; and (c) women (but not men) display a significant relationship between marital dissolution and promiscuity, suggesting that female
sexual exclusivity is an important criterion in male choice. These results are discussed in relation to understanding mate
choice mechanisms from behavioral data.
Daniel Pérusse is an assistant professor in the Department of Anthropology at the Université de Montréal. His research interests
include the evolutionary biology of human social and reproductive behavior, sexual selection theory, and biocultural evolution.
His current research bears on human socialization processes and psychosocial development from an evolutionary and behavior-genetic
perspective. Recent publications include “Cultural and Reproductive Success in Industrial Societies: Testing the Relationship
at the Proximate and Ultimate Levels” (Behavioral and Brain Sciences 16(2):267–322, 1993) and “Human Parental Behavior: Evidence for Genetic Influence and Potential Implications for Gene-Culture
Theory” with M. C. Neale, A. Heath, and L. J. Eaves (Behavior Genetics, in press). 相似文献
39.
PAULO ANDREAS BUCKUP 《Zoological Journal of the Linnean Society》1993,108(3):225-245
Although virtually no phylogenetic evidence (in the sense advocated by Hennig, 1966) had been previously presented to support the monophyly of the Characidiinae, and most 'diagnostic' characters used by previous authors were found to be unacceptable in a cladistic classification, i t is still possible to diagnose the Characidiinae in a phylogenetic sense. This study revealed the existence of 13 synapomorphies supporting the monophyly of the group. Several of these synapomorphies, such as the modifications associated with the mesethmoid, the jaw bones, and the ribs of the fifth vertebra, are unique to the Characidiinae, thus providing a solid basis for recognizing the group as a monophyletic unit of characiform fishes. Demonstration of characidiin monophyly provides a solid foundation for further phylogenetic analysis of characidiin interrelationships, and higher level relationships among characiform fishes. 相似文献
40.
Nicholas Shea 《Biology & philosophy》2007,22(3):313-331
There is ongoing controversy as to whether the genome is a representing system (Sterelny K., Smith K.C. and Dickson M. 1996.
Biol. Philos. 11: 377–403; Griffiths P.E. 2001. Philos. Sci. 68: 394–412). Although it is widely recognised that DNA carries
information, both correlating with and coding for various outcomes, neither of these implies that the genome has semantic
properties like correctness or satisfaction conditions (Godfrey-Smith P. 2002. In: Wolenski J. and Kajania-Placek K. (eds),
In the Scope of Logic, Methodology, and the Philosophy of Sciences, Vol. II. Kluwer, Dordrecht, pp. 387–400). Here a modified
version of teleosemantics is applied to the genome to show that it does indeed have semantic properties – there is representation
in the genome. The account differs in three respects from previous attempts to apply teleosemantics to genes. It emphasises
the role of the consumer of representations (in addition to their mode of production). It rejects the standard assumption
that genetic representation can be used to explain the course of an organism’s development. And it identifies the explanatory
role played by representational properties of the genome. A striking consequence of this account is that other inheritance
systems could also be representational. Thus, a version of the parity thesis is accepted (Griffiths P.E. 2001. Philos. Sci.
68: 394–412). However, the criteria for being an inheritance system are demanding, so semantic properties are not ubiquitous. 相似文献