Vestigial organs Structures that are of no use to the possessor, and are not necessary for their existence are called vestigial organs. Vestigial organs may be considered as remnants of structures which were well developed and functional in the ancestors, but disappeared in course of evolution due to their non- utilization. Human appendix is the remnant of caecum which is functional in the digestive tract of herbivorous animals like rabbit. Cellulose digestion takes place in the caecum of these animals.
Due to change in the diet containing less cellulose, caecum in human became functionless and is reduced to a vermiform appendix, which is vestigial. Other examples of vestigial organs in human beings )LVK 6DODPDQGHU 7RUWRLVH &KLFN +XPDQ 3KDU\QJHDO SRXFKHV 3RVWDQDO WDLO 6DODPDQGHU &KLFN 3KDU\QJHDO SRXFKHV Fig . Embryological evidences XII Std Zoology Chapter XII Std Zoology Chapter Evolution theory of recapitulation” which states that the life history of an individual (ontogeny) briefly repeats or recapitulates the evolutionary history of the race (phylogeny). In other words “Ontogeny recapitulates Phylogeny”.
The embryonic stages of a higher animal resemble the adult stage of its ancestors. Appearance of pharyngeal gill slits, yolk sac and the appearance of tail in human embryos are some of the examples (Fig. . ).
The biogenetic law is not universal and it is now thought that animals do not recapitulate the adult stage of any ancestors. The human embryo recapitulates the embryonic history and not the adult history of the organisms. The comparative study of the embryo of different animals shows structural similarities among themselves. The embryos of fish, salamander, tortoise, chick and human start life as a single cell, the zygote, and undergo cleavage to produce the blastula, change to gastrula and are triploblastic.
This indicates that all the above said animals have evolved from a common ancestor.