function Animal(genus, species) {
    var animal = Object.create(Animal.prototype);
    animal.genus = genus;
    animal.species = species;
    return animal;
}


function Dog(species) {
    var dog = Object.create(Dog.prototype);
    dog.genus = 'Canis';
    dog.species = species;
    return dog;
}
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;