一个关于继承问题

一个关于继承问题
Public MustInherit Class animal
'名字
Public name As String
'可由子类从新定义的方法
Public Overridable Sub shout()
System.Console.WriteLine("某种动物在叫")
End Sub
End Class
Public Class dog
Inherits animal
Public Overrides Sub shout()
System.Console.WriteLine("汪汪。。。，我叫{0},我会看门，同时兼职抓耗子以赚点钱", name)

End Sub
Public Class cat
Inherits animal
Public Overrides Sub shout()
System.Console.WriteLine("喵喵。。。。，我叫{0},只要会讨好主人，吃穿不愁", name)

End Sub
End Class
End Class
Module Module1

Sub Main()
'创建一只狗，起名”威虎”
Dim d As New dog
d.name = "威虎"
d.shout() '狗在做自我介绍
'创建一只猫，起名“小妙”
Dim c As New dog.cat

c.name = "小妙"
c.shout() '猫在做自我介绍
Dim a As animal
a = d
a.shout()
a = c
a.shout()
'暂停
System.Console.ReadLine()

End Sub

End Module

上为什么Dim c As New dog.cat不能直接用Dim c As New cat呢?