56/63---3题

cdg2004

Passage 56

     Although numbers of animals in a given region may

   fluctuate from year to year, the fluctuations are often

   temporary and, over long periods, trivial. Scientists

   have advanced three theories of population control to

(5) account for this relative constancy.

   The first theory attributes a relatively constant popu-

   lation to periodic climatic catastrophes that decimate

   populations with such frequency as to prevent them

   from exceeding some particular limit. In the case of

(10) small organisms with short life cycles, climatic changes

   need not be catastrophic: normal seasonal changes in

   photoperiod (daily amount of sunlight), for example,

   can govern population growth. This theory---the

   density-independent view---asserts that climatic factors

(15) exert the same regulatory effect on population regard-

   less of the number of individuals in a region.

      A second theory argues that population growth is

   primarily density-dependent---that is, the rate of

   growth of a population in a region decreases as the

(20) number of animals increases. The mechanisms that

   manage regulation may vary. For example, as numbers

   increase, the food supply would probably diminish,

   which would increase mortality. In addition, as Lotka

   and Volterra have shown, predators can find prey more

(25) easily in high-density populations. Other regulators

   include physiological control mechanisms: for example.

   Christian and Davis have demonstrated how the

   crowding that results from a rise in numbers may bring

about hormonal changes in the pituitary and adrenal

(30) glands that in turn may regulate population by lowering

   sexual activity and inhibiting sexual maturation. There

   is evidence that these effects may persist for three

   generations in the absence of the original provocation.

   One challenge for density-dependent theorists is to

(35) develop models that would allow the precise prediction

   of the effects of crowding.

      A third theory, proposed by Wynne-Edwards and

   termed “epideictic,” argues that organisms have evolved

   a “code”in the form of social or epideictic behavior

(40) displays, such as winter-roosting aggregations or group

   vocalizing; such codes provide organisms with infor-

   mation on population size in a region so that they can,

   if necessary, exercise reproductive restraint. However,

   wynne-Edwards’ theory, linking animal social behavior

(45) and population control, has been challenged, with some

   justification, by several studies.

第三题答案是A,

3. Which of the following, if true, would best support the

  density-dependent theory of population control as it is

  described in the passage?---类比题

  (A) As the number of foxes in Minnesota decrease, the

     growth rate of this population of foxes begins of

     increase.---密度理论：种群数量增加，增长就会减慢；反之亦然？即当种群数量减少时，物种增长的速度就会加快？

  (B) As the number of woodpeckers in Vermont

     decreases, the growth rate of this population of

     woodpeckers also begins to decrease.

  (C) As the number of prairie dogs in Oklahoma

     increases, the growth rate of this population of

     prairie dogs also begins to increase.

  (D) After the number of beavers in Tennessee decreases,

     the number of predators of these beavers begins to

     increase.

  (E) After the number of eagles in Montana decreases,

the food supply of this population of eagles also

begins to decrease.

问题：选项不是直接与文章内容进行类比，而是用反命题方式来支持文章中的原命题。

原命题与逆否命题可以对等，但原命题可以与否命题对等吗？

[此贴子已经被作者于2005-4-7 14:36:07编辑过]

而今迈步从头越

这和MM问的55/63---5T从逻辑的角度讲是完全不一样的类型：

55/63---5T是infer题，得出的conclusion必须是能从文中infer出来的，因此只能是逆否命题或者从原文能够推出；而本题是support，只要能support原文结论就行，而由非Aà非B来加强AàB是一种典型的加强方式（当然这种不能作为assumption的答案）。

附55/63—5T问题：observers mentioned in line 1 would be most likely to predict which of the following?

cdg2004

很高兴MM这么快答复，这正是一箭双雕，一问解两题了！

另外，什么是非Aà非B？中间小a还加一点，是什么意思呀？

追逐梦想2006

解码： 非Aà非B 即 非A --> 非B

乱码而已。