Declaring classes that inherit from another class

The following lines show the code for the Animal base class in Swift. The class header doesn't specify a base class, so this class will become our base class for the other classes. The code file for the sample is included in the swift_3_oop_chapter_04_01 folder:

    open class Animal { 
      open static var numberOfLegs: Int { 
        get { 
          return 0;           
        } 
      } 
      open static var averageNumberOfChildren: Int { 
        get { 
          return 0; 
        } 
      }     
      open static var abilityToFly: Bool { 
        get { 
            return false; 
        } 
      } 
     
      open var age: Int      
      init(age : Int) { 
        self.age = age 
        print("Animal created") 
      } 
     
      open static func printALeg() { 
        preconditionFailure("The pringALeg method...