Java Generics Silly Thing (Why cant I infer the type?)

I,m assessing using Generics for a behavior. I managed to get it work but I don,t understand why cant I infer the type from the arg. So I created this example which runs ok without warnings with the purpose of simplifying the actual architecture. Suppose I got these two classes. The target one: package Zoo; import Zoo.Main.CageFamily; import Zoo.Main.Vertebrate; public class ZooCage