Whenever a species with an organ fully developed is crossbreedable with any other species showing the same organ vestigialized, this capability of hybridization can be taken as an indication for vestigialization that actually happened. This is the case with the cave characin and the seeing variety, that can be crossed without any problems (see pic. 2, pic. 78). The human wisdom teeth are a further typical example. It is most likely, that originally all 32 teeth were functioning and needed. That the wisdom teeth are not necessarily needed anymore today, might be due to a change in human eating habits. A degenerative development of these molars was not of any disadvantage and therefore possible. The vestigialization of the wisdom teeth is different from person to person. A third example are flightless insects that live on stormy islands (see Mutation and pic. 58).
In the case of the whales the criteria of hybridization capability is not effective. There is too big a difference between whales and their presumed land-living ancestors. How then can vestigialization be identified in cases like these? There are mainly two criteria:
1. Uselessness. If it could be proven that an organ is totally useless and has no function at all this could be seen as a hint for the interpretation that the organ once had a function, which now got lost. The abdominal bones of the whales however cannot be called vestigial this way, for they do have a very important function: They form the base for the genital muscles, possibly as well for the elevator muscle of the anus and furthermore support abdominal wall and viscera. Anyway, uselessness is difficult to verify. It can often only be said, that a function is unknown – not, that there is none at all. In many cases, the vital or at least useful function of organs has been discovered long after describing them as useless. This is why the argument of uselessness is problematic and not well founded.
In their studies, the American scientist William Parker and his team came to the conclusion that the human appendix represents a kind of safe house and rescue post for symbiotic bacteria, that supports the increase of useful intestinal bacteria and enables or promotes a repopulation after diarrhea. These bacteria prevent the spreading of dangerous pathogens in the human digestive tract, which is especially important after diarrhea. Details are explained in The appendix as safe house.
Inconsistencies between structure and function. A modified argumentation says that vestigial organs indeed do have a function – but that this function is too insignificant compared to the structural expense. This kind of inconsistencies between structure and function are hardly verifiable, as well.