It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. But left- and right-handed snails are not actually true mirror images of one another…
But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant…
Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness…Thus, the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself。
是什么样的原因致使自然界中的螺旋体呈现出右旋(“右向旋转的”，或顺时针的)或左旋(“左向旋转的，”或反时针的)?这是形态科学中一个最引人入胜的不解之谜....。。究竟是一些什么样的机制控制着旋转方向，并使左旋的比例为数稀少呢?
假如左旋和右旋蜗牛呈完全一致的镜像对称的话，那么进化对左旋蜗牛不利，这近乎不太可能。几乎无法想象左旋本身会给左旋者带来任何不利之处。然而，左旋与右旋蜗牛彼此间实际上并不呈真正的镜像对称....。。
但是，这种结合不对称、解剖学和偶然性的进化论机制并未提供一种充分的解释，以说明蜗牛的右旋何以会成为主要的旋向....。。
于是，进化论必须让位于支持右旋或左旋的明确的发育机制为基础的理论....。。因此，解决所有蜗牛身上旋向之谜的道路似乎与这一螺旋体本身一样曲折复杂。
上文在第一段最后提出一个问题：“究竟是一些什么样的机制控制着旋转方向，并使左旋的比例为数稀少呢?”这个问题就是一个现象：螺旋体中左旋的比例少。有问题就有解答，有现象就有解释。第二段从进化论的角度解释这一现象。但是，GRE阅读文章中一般对进化论的解释持负面评价，所以需要寻找新的解释。第三段否定了进化论的解释。在末段，作者以发育机制(developmental mechanism)解释“为何左旋少、右旋多”。
把握了文章的三种主体结构以后，做文章后的许多题时易如反掌。以上述的“结论——解释型”文章为例，如果熟悉文章的主体结构，下面两道题就很容易找到正确答案。
1. The passage is primarily concerned with
(A) describing an important technique
(B) discussing a new method
(C) investigating a controversial procedure
(D) announcing a significant discovery
(E) promoting a novel application
正确答案对应文章首句的主题句。因此(A)为正确答案。
2. Which of the following best describes the organization of the passage?
(A) A method is criticized, and an alternative is suggested。
(B) An illustration is examined, and some errors are exposed。
(C) An assertion is made, and a procedure is outlined。
(D) A series of examples is presented, and a conclusion is drawn。
(E) A hypothesis is advanced, and supporting evidence is supplied。
本文的结构是“结论—解释型”，因此(C)为正确答案。
由此可见，把握文章的主体结构，而不陷入文章词句与细节的泥沼中，是一件快意的事情：文章读得爽，题也做得爽。
