He beat more furiously than ever upon the cage and slipped his great fingers through the bars, trying to reach the Professor, and poured out volumes of ape-chatter.
Why, why did men shoot at him, he asked. He made himself terrible, therefore men ought to love him. That was the whole burden of what the Professor calls its argument.
This is a novel about a gorilla who could speak with a professor. I do not understand the meaning of the sentence below.
That was the whole burden of what the Professor calls its argument
What does "burden" mean in this context?
Does "argument " mean "a quarrel" here?