97．They are an example of a common theme in evolution，the more or less paralleldevelopment of different types of body structure and function for the samereason--in this case，for flight．

(the more or less parallel development ofdifferent types of body structure and function for the same reason作a common theme in evolution的同位语)

它们例证了进化中的一个常见主题，即不同类型的身体结构和功能为了同一目的而或多或少地平行发展——在这种情况下，是为了飞行的目的。

分句1：They are an example of a commontheme in evolution

分句2：the more or less paralleldevelopment of different types of body structure and function for the samereason

分句3：--in this case，for flight．

2和3嵌套，3是2的补充说明

1和2嵌套，2是1中theme的同位语