Variance under inheritance

One of the ways we can learn about a concept is to ask questions that lead you to the concept. So let's ask a question ourselves. Given that a Cat class type extends the Animal class, is it okay to treat a list of cats as a list of animals? Programmatically, see the following:

abstract class Animal() 
 
class Cat(name: String) extends Animal()               // Inheritance relationship between Cat and Animal 
 
def doSomethingForAnimals(animals: List[Animal]) = ??? //definitely do something for animals. 
 
Is it possible to pass an argument that's a list of Cats? 
val cats = List(new Cat("Mischief"), new Cat("Birdie")) 
doSomethingForAnimals(cats) 

If it's possible, the statement that List[Cat] is a subtype of List[Animal] makes sense. Also, the concept is called covariance. Hence, we say List is covariant in its type parameter T:

If you take a look at the preceding image, the direction of the inheritance relationship between two concrete classes, namely Cat and Animal...