听力文本

But transitional forms are also the rarest of beasts.
不过过渡型物种也是最罕见的动物
By their very nature they were few in number and lived for just a short intermediate time until a wholly new animal evolved.
从它们的性质不难看出，它们不仅数量少，而且只生活在非常短暂的过渡时期，直到进化成全新的物种
In fact for years there was only one accepted transitional form - the archaeopteryx
事实上，多年来只有一种认可的过渡型物种--始祖鸟
which was a dinosaur with feathers that marked the transition to birds.
一只长羽毛的恐龙，标志着向鸟类的过渡

Presumably the transitional forms are very rapidly outcompeted by their more, by their own more advanced descendants
大概是因为过渡型物种很快即被它们自己更先进的后代所取代
so these transitional episodes in the history of life tend to be brief
使得这些生命历史中的“过渡插曲”相当短暂
and involve, it seems, relatively low numbers of species and probably low numbers of individuals.
包括这些动物的物种数量可能比较少，种内数量也可能非常少
The trouble was without anything half fish/half tetrapod between eusthenopteron and ichthyostega
问题是，真掌鳍鱼和鱼石螈之间若是找不到半鱼半四足类的动物
the story of how we got our legs remained incomplete and lurking out there
那么关于我们如何长出腿的故事就会变得很不完整
was a group dedicated to pointing out such gaps in the story of evolution with the aim of its ultimate destruction.
那儿总有一些人，一心想要找出进化理论中的这类破绽，直到它最后整个坍塌。

词汇解析

1.tend to be 倾向于；趋向于
We also tend to be similar to our friends and family on income and wealth measures.
我们在收入和财富衡量标准方面也趋向于我们的朋友和家庭成员。

2.be dedicated to 献身于；致力于
As a scientist, he was dedicated to the truth.
作为一个科学家，他献身于追求真理。

3.with the aim of 以…为目标；其目的是为了...
I am here with the aim of doing as well as possible.
我来这儿的目的是尽可能做到最好。

视频及简介

19世纪，英国生物学家赫胥黎在研究了尼安德特人的头骨以后，认为是一个具有猿脸的头骨，但决不是人与猿之间的中间缺环。这就告诉我们，人与猿之间的亲缘关系，似乎还应该有一环。1891年发现直立猿人（爪哇猿人）的骨胳，证明这个缺环确实是存在的。可是，日本的人类学家认为爪哇猿人尚不是真正的中间缺环。那么，在猿与人之间到底应该是什么样的一环呢？在埃塞俄比亚发现的早期人科动物化石，很可能是过渡物种。这一发现，弥补了原始人类进化中的一个“缺环”。