i guess gumball just overfed him until one दिन he grew legs. i don,t really know but they should have an episode where we can see darwin growing his legs.
The is just one episode that is विभाजित करें, विभक्त करें into two bits Because gumball was ADHA as 4 साल old. He was दिया Darwin when he was accidently flushed down the toilet. But when he was about to die on dry land gumballs प्यार for घर made him evolve into having lungs this is also the reason why Darwin has legs.
