why do some camels have one hump and others have two?

Question: why do some camels have one hump and others have two?
- Keywords:
  camel,
  hump
Asked by livgn1029 to Jo, Kevin, Louise, Valeria on 20 Jun 2012.
- Kevin Mahon answered on 20 Jun 2012:
  
  They are different species!
  
  The Dromedary (one hump) and the Bactrian Camel (which has two!)
  
  The Bactrian Camel is much rarer than the Dromedary – there are only two species of camel currently!

Question: why do some camels have one hump and others have two?