His studies of ice-polished rocks in his Alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets existed in what are now temperate areas.

"led" is a discrete event in the past, so an earlier event would take the past perfect. Therefore , should we not use had existed in above sentence?.... If not, Why ?

Thank you
Some people do say that if there are two past (actions)(events), the earlier one should be expressed with the past perfect. But if the meaning is clear without using the past perfect, as it often is, the past perfect is in fact not necessary. That is the case in your sentence.
Hello Pokh,

You are correct in that we have two levels of the past here in this sentence. However, it is irrelevant which one came before the other in this case because the sentence is very detailed in its description of timing. We are even given the year when Louis Agassiz proposed his concept. So therefore there is no ambiguity in the sentence as it is well understood that the ice age must have predated his concept of an ice age or the expedition. So "had existed" would sound redundant in this case.

Here is a quote from the Wikipedia page on English Verbs:

It is sometimes possible to use the simple past instead of the past perfect, but only where there is no ambiguity in the meaning.

Here is the link also:

http://en.wikipedia.org/wiki/English_verbs

-Chris
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pokhan age in which great ice sheets existed in what are now temperate areas.
The comparison is between the ice ages and 'now', so a past perfect doesn't seem appropriate. You use the past perfect to show that something happened before a reference point in the past, not that it happened before 'now'.

Note the importance of the difference in time scales. On one time scale you have Agassiz's lifetime. On another, larger time scale you have geological ages in which glaciers may form and melt and reform. On the larger scale, Agassiz's lifetime and 'now' are essentially the same time. (See diagram below.)

If the comparison were otherwise, with a time reference ('then') very long ago, well before Agassiz's birth, there might have been some justification for a past perfect:

an (even ealier) age in which great ice sheets had existed in what were then temperate areas.

If you want a past perfect on the scale of Agassiz's lifetime, you could have:

This led him to propose the concept of ice ages, an idea he had rejected earlier in his life.

When, as very rarely happens, you have a sentence that presents facts on two different time scales, you can't normally mix the two scales and justify the past perfect on one time scale because it precedes the past on the other time scale.

..[ice ages]................... [ now ]..............

...................................[Agassiz's life.......our lives].............

CJ