And this is where arbitrariness inevitably enters into the selection. Of all scholarly disciplines history is the most unscientific, because it possesses or can possess least of all an assured, approved method of selection; that is, critical research has a very definite method, but the presentation of it has not.
就是在这里，任意性不可避免地进入了选择中。所有学科中历史是最不科学的了，因为它最不具有或最不能够具有一种确定的，公认的选择方法；也就是说，批判的研究有非常明确的方法，但对它的呈现却没有。
It is on every occasion the record of what one age finds worthy of note in another.
【这段完全看不明白】
Every historian will have a special selection, a different criterion for what is worth communicating, according to his nationality, subjectivity, training, and period.
根据他的国籍、主观、训练和时代，每个历史学家都将有特殊的选择，对什么值得传达有不同的标准。
參考答案:它(指历史)在每个场合是一个时代发现的对自身有价值的记录.
我觉得是这个意思