全文获取类型
收费全文 | 5841篇 |
免费 | 720篇 |
国内免费 | 131篇 |
出版年
2024年 | 14篇 |
2023年 | 73篇 |
2022年 | 57篇 |
2021年 | 142篇 |
2020年 | 189篇 |
2019年 | 208篇 |
2018年 | 186篇 |
2017年 | 237篇 |
2016年 | 270篇 |
2015年 | 321篇 |
2014年 | 326篇 |
2013年 | 345篇 |
2012年 | 262篇 |
2011年 | 285篇 |
2010年 | 283篇 |
2009年 | 337篇 |
2008年 | 323篇 |
2007年 | 306篇 |
2006年 | 240篇 |
2005年 | 254篇 |
2004年 | 191篇 |
2003年 | 189篇 |
2002年 | 202篇 |
2001年 | 173篇 |
2000年 | 137篇 |
1999年 | 135篇 |
1998年 | 130篇 |
1997年 | 109篇 |
1996年 | 100篇 |
1995年 | 82篇 |
1994年 | 69篇 |
1993年 | 79篇 |
1992年 | 65篇 |
1991年 | 56篇 |
1990年 | 27篇 |
1989年 | 39篇 |
1988年 | 25篇 |
1987年 | 30篇 |
1986年 | 23篇 |
1985年 | 30篇 |
1984年 | 32篇 |
1983年 | 18篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 12篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有6692条查询结果,搜索用时 15 毫秒
21.
22.
23.
24.
The effects of ethylene oxide (EtO) on survival of soil microflora and on selected chemical properties of a sandy-loam soil
were examined.
Soil sterilization was achieved after 8 hours exposure to EtO. Ethylene oxide treatment increased soil pH and organic matter
content. Extractable Mn and Fe slightly increased whereas P content decreased. Total N was not affected by the treatment. 相似文献
25.
A. G. M. Steerneman E. A. van der Meulen W. Schaafsma G. N. van Vark 《Human Evolution》1990,5(5):449-456
Research on human evolution and sexual dimorphism motivates an interesting test problem. In studying hominid phylogeny it
is of interest to test whether parallel evolution plays a role. With regard to sexual dimorphism it is of interest to known
whether the directions of sexual dimorphism in the populations being compared are the same. We show that testing these two
problems gives rise to the same type of hypothesis testing, viz. the problem of testing the hypothesis that the means of independent,
normally distributed random vectors with unit covariance matrices are situated on a straight line through the origin. A test
is proposed and applied to study the sexual dimorphism of 20 recent skull populations. In this example the hypothesis of equal
directions of sexual dimorphism is rejected. The classical theory of constructing multiple discriminant functions (canonical
variates) is adapted to the problem of comparing sexual dimorphisms. 相似文献
26.
To analyze the sexual behavior of male black-legged deer ticks Ixodes dammini,we collected ticks infesting 202 white-tailed deer. On average, 17.7 males and 8.8 females infested each deer. Field-collected males copulated with a mean of 2.25 females, and virgin males mated with 2.4 females. On experimental hosts, males established sexual contact with feeding females and repelled other males, and about half remained paired after their mate detached. Engorged females continue to be receptive, and males mate more readily with them than with nonfed females. We conclude that male I. damminiare endowed with a repertoire of behaviors which favor an opportunistic mating before seeking a host and a preference for mating with feeding females on the host accompanied by tenacious mate guarding. 相似文献
27.
Mating behavior in seaweed flies (Coelopa frigida) 总被引:1,自引:0,他引:1
28.
Urs Kägi James G. Chafouleas Anthony W. Norman Dr. Claus W. Heizmann 《Cell and tissue research》1988,252(2):359-365
Summary Calcium and intracellular Ca2+-binding proteins are possibly involved in hormone production and spermatogenesis in rat testis. Parvalbumin, calbindin D-28K, S-100 proteins and calmodulin were localized in the Leydig cells, which are sites of testosterone synthesis. Only the appearance of parvalbumin-immunoreactivity is closely correlated to testosterone production during development of the testes. Calbindin D-28K-immunoreactivity persisted in foetal-type Leydig cells and in adult-type Leydig cells at all stages of development. S-100-immunoreactivity was low during all foetal stages, absent between birth and puberty, and increased thereafter. Calmodulin staining is most prominent in the cytoplasm of developing spermatocytes and of maturing spermatids. All four proteins co-exist in the seminiferous tubules. The distinct localization and developmental appearance of these proteins suggests different regulatory roles in Leydig cell function and spermatogenesis. 相似文献
29.
Menstrual-cycle phase and sexual behavior in semi-free-ranging stumptail macaques (Macaca arctoides)
The sexual behavior and female reproductive cycles of a group of island-dwelling stumptail macaques (Macaca arctoides)were monitored over a 6-month period, yielding 530 observation hr and 268 copulations. Compared to nondominant males, the
dominant male copulated at a relatively high rate throughout the cycle, but largely with one high-ranking female. The non-dominant
males copulated most frequently at midcycle. Female presenting was highest at midcycle, but only to the dominant male. Cross-study
discrepancies may be due to different observation methods and restricted environmental conditions that mask female-initiated
sexual behavior. The more naturalistic setting of this study allowed for a fuller expression of proceptivity. Contrary to
some previous conclusions, present findings suggest that both hormonal and socioenvironmental factors influence the patterns
of sexual behavior found in stumptail macaque colonies. 相似文献
30.
Patterns of variation in tail ornament size in birds 总被引:5,自引:0,他引:5
RAUNO V. ALATALO JACOB HÖGLUND ARNE LUNDBERG 《Biological journal of the Linnean Society. Linnean Society of London》1988,34(4):363-374
In recent years several different kinds of sexual selection models have been developed, and tail ornaments in birds have frequently been used as an example of a sexually selected character where the models might apply. However, very little is known about intra- and interpopulation variation in ornament size. We have studied the elongated tail ornaments in four species of whydahs Vidua , the forktailed flycatcher Tyrannus savana and the Asian paradise flycatcher Terpsiphone paradisi. Ornaments were relatively longer in males with the longest tarsi ('heterogony' with positive allometry). Also, tail lengths were remarkably variable within each geographical area, the coefficient of variation (average = 11%) being three times as high as for body size characters. Models, with female preference of ornaments bearing no relation to male viability, usually generate lines of neutral equilibria. Thus, they predict extraordinary variation in ornaments between populations. However, elongated tail ornaments did not show higher geographical variation than the body size characters, suggesting that there is no line of equilibria for these ornaments. 相似文献