Key Measurements

[Joost Romeu]

Why are measurements so important to identification?

I can understand why quantities (e.g. 5 stamens, 6 fingers) and relative
measurements (e.g. petals smaller than sepals) can be definitive but have a
difficult time understanding the importance of measurements for the following
reasons:

1) Don't measurements vary based on the age/growth stage of the specimen? If so,
should all measurements be averages?

2) Isn't any measurement necessarily (because of difficulties measuring using
analog instruments such as rulers, for example) approximate?

3) If three things are identical in every way but one is twice the size of the
first; and the other three times the size of the first would there be cause to
say that each thing being identified might be identified as a unique thing
(using "thing" broadly).

Do measurements define taxa or do they just confirm taxa that have been narrowed
down in other ways?