But in "proving" evolution in which there are no specific species that are any more "transitional" than any others, a "doubter" demanding to see such a "transitional" critter is pre-imposing constraints on nature that may or may not exist.
You are never going to get grandpa, grandma, pa, ma, son, daughter, grandson, granddaughter fossils -- all lined up nice in a row -- well at least the probabilities are against it.
What you get is random snapshots in time. There are plenty of examples of incremental changes in the fossil record and these have been posted to these threads all the time.
But of course, since these snapshots are always going to be specific species, one can always ask, "where is the proof", "where is the transistion between this one and the previous."
As someone pointed -- each "transitional" species found opens up two new "gaps". The seekers of "transitional" species simply cannot be satisfied because their actual definition is vague and arbitrary -- the status they seek, "transitional species" is not a reality. All species are at some "transitional" stage between what they were and what they will become.