Dollo's rule 2-2

Dollo's rule

Louis doll Belgian paleontologist in 1893 propose that an organism never returns exactly to a former state, even if it finds itself placed in conditions of existence identical to those in which it has previously lived. moreover, it always keeps some trace of the intermediate stages through which it has passed. 

often it is misinterpreted that old organs cannot grow back or evolution cannot be reversed. Goldberg 2008. but the rule really means is that there is statistical improbability to trace the same path of evolution twice.  

it also applies to molecular features also. eg. humans lost gulonolactone oxidase, the final enzyme in the biosynthetic pathway of vitamin C. yang 2013. but despite requirement it is not returned. 

but this does not mean that species cannot adapt to changing environment

white-throated rail bird show the phenomena of iterative evolution which is repeated evolution of similar or parallel structures from ancestors. 

mammals like whales and dolphins are aquatic animals but during evolution mammals evolved lungs from gills and it cannot be reversed.  similarly bats evolved flying with anatomy of hands not wings.