While Darwin didn't have the detailed information regarding genetics, so I don't know why you think it's a pertinent question to ask in a "Darwinian" sense.   We know that a significant percentage of "transitions" (if you want to call them that) maybe tiny variations that may only be detectable at the genetic level.  In many cases, changes could occur that don't express and may only express during particular stressors.  In other cases, variations may occur that are phenotypically undifferentiated, and may even be lost because no advantage is conferred.

To suggest that each variation results in a "transition" implies that there is a directionality to evolution which simply isn't there.  More to the point, you may not even recognize a transition when it occurs, because it may not be phenotypic as much as it is adaptive.  As a result, it's entirely possible for transitions to have occurred that wouldn't necessarily be reflected in the fossil record.

There have been special instances of individuals that have adapted using their feet for painting, writing, etc. when they are absent arms.  To examine such individuals from a purely skeletal examination (or imagining a fossil record) their adaptation of using feet would be undetectable.

Asking about "transitions" only concentrates on external changes, which may not necessarily be the most significant.  How would you examine the change in lung usage and metabolism for aquatic mammals being able to hold their breath for long periods?  or are you only concerned with fins?