No polymorphism?

namespace Game {
    class Stage {
        const int UNIT_H = 101;
        const int UNIT_V = 82;
        private Clutter.Stage _clStage;

        public Stage () {
            _clStage = Clutter.Stage.get_default ();
            AddTree (5,3);

            _clStage.color = Clutter.Color () { alpha = 255 };
            _clStage.show_all ();
        }

        public void AddDrawable (ref Drawable drawable) {
            _clStage.add_actor (drawable);
        }

        public void AddTree (int x, int y) {
            Tree t = new Tree (x*UNIT_H, y*UNIT_V+UNIT_V/2);
            AddDrawable (ref t);
            /* Error: Cannot convert from `Game.Drawable' to `Game.Tree?'
                    AddDrawable (ref t);
                                 ^^^^^ */
        }
    }


    class Drawable : Clutter.Actor {
        private Cairo.ImageSurface _surface;

        public Drawable (float x, float y, float width, float height, string filepath) {
            this.x = x;
            this.y = y;
            this.width = width;
            this.height = height;

            var canvas = new Clutter.Canvas ();
            set_content (canvas);
            canvas.set_size ((int)width, (int)height);
            canvas.draw.connect (drawme);

            _surface = new Cairo.ImageSurface.from_png (filepath);
            canvas.invalidate ();
        }

        private bool drawme (Cairo.Context ctx, int w, int h) {
            ctx.set_source_surface (_surface, 0, 0);
            ctx.paint ();
            return true;
        }
    }

    class GameObject : Drawable {
        const int IMG_WIDTH = 101;
        const int IMG_HEIGHT = 171;

        public GameObject (float x, float y, string filepath) {
            base (x, y, IMG_WIDTH, IMG_HEIGHT, filepath);
        }
    }

    class Tree : GameObject {
        const string IMG_PATH = "imgs/Tree Tall.png";
        public Tree (float x, float y) {
            base (x, y, IMG_PATH);
        }
    }
}