For those who don't know, otters are mammals. And since we can quickly see that the premises are both true, while the conclusion is false, this argument is invalid. Although it may be used to demonstrate the invalidity of argument forms, it cannot be used to demonstrate their validity. Failure to produce a counterexample to an argument form could be a sign that the form is valid. But it is also possible that the problem simply lies in the creativity required to construct a substitution instance of the form.
All mammals are animals.
So, all cats are animals. But notice that we can just as easily classify this argument with another form. George W. Bush was Governor of Texas. So, David Lewis is the 43 rd President of the United States. What we have shown is that arguments may have valid and invalid argument forms.