In addition, the ontogeny of an individual organism yeilds clues about the ancestral species it evolved from. Evolutionary biologists would say this is because mammals evolved from organisms that had gills. In order for modern day mammals to develop, they must pass through a stage where the gills begin to form in order to get to their present form. Present day organisms are modified descendents of their ancestors. Traces of their ancestry cannot be erased from ontogeny because there is no evolutionary mechanism for starting an organism from scratch.
Natural selection can only modify existing traits of an organism.
Since changes early on are more likely to be deleterious because all subsequent development is dependent on early growth, organisms pass through stages of development that are reminiscent of the stages of develop ment of their ancestors. Remnants of an organisms phylogenetic history that serve no present purpose are called vestigal structures. A good example of a vestigal structure are the rudiments of legs still seen in whale skeletons. The only difference is that they are scaled differently. Evolutionary biologists say this indicates that all mammals are modified descendents of a common ancestor.