Missing links is a term used in the Paleontology that refers to the fossil record(s) that is yet to be found and that is needed to connect two evolutionary stages. Missing links is the Achilles heel of the theory of evolution.
Secondly, while Dinosaurs allegedly evolved into birds, and some gerbil -sized mammals gave rise to the big cats, why cockroaches, mosquito, and flys did not evolve? They all have been found preserved in he amber resins from the same and older time periods. Horseshoe crabs for instance haven't evolved a bit when compared their present forms with their tens of thousands of years old fossils. Archea and bacteria divide (on average) every 15-20 minutes, that is some 72 times in 24 hours and a single bacterium would give rise to 4,722,366,482,869,645,213,696 bacteria after 24 hours. With such a high generation turn over why do they fail to evolve into more intelligent forms? These are the questions I ask to myself as a Molecular Biologist.