Ammonia is so toxic that it can be transported and excreted only in large volumes of very dilute solution. As a result most terrestrial animals simply don't have access to sufficient water to routine excrete ammonia.
The main advantage of urea is its very low toxicity. Animals can transport urea in the circulatory system and store it safely in high concentrations. Also much less water is loss when a given quantity of nitrogen is excreted in a concentrated solution of urea than would be in a dilute solution of ammonia.
ONE OF THE OLDEST ARGUMENTS IN THE HISTORY OF PSYCHOLOGY IN THE NATURE VERSES NURTURE DEBATE EACH OTHER THESE SIDES HAVE GOOD POINTS THAT IT IS REALLY HARD TO DECIDE WHETHER A PERSON'S DEVELOPMENT IS PREDISPOSED IN HIS/HER DNA, OR A MAJORITY OF ITS INFLUENCED BY THIS LIFE EXPERIENCES AND HIS/HER ENVIRONMENT .........
