Institutio Oratoria

On the other hand rhetoric, which derives its name from the power of eloquence, must not shirk its peculiar duties nor rejoice to see its own burdens shouldered by others. For the neglect of these is little less than a surrender of its birthright.

I will of course admit that there may be a few professors of literature who have acquired sufficient knowledge to be able to teach rhetoric as well; but when they do so, they are performing the duties of the rhetorician, not their own.