What is the main metaphor in D.H. Lawrence's poem "Lesson of the Afternoon"?

The main metaphor in D. H. Lawrence’s poem “Last Lesson” is a comparison between his students and a pack of dogs. In the first stanza of the poem, the narrator (who is a teacher) compares his students to hunting dogs. Hunting dogs will only pursue the things they are instinctively drawn to. As a teacher, he feels helpless and questions his ability to spark an interest in learning in his pupils. The students are not drawn to education and cannot be forced to absorb information when they act like the hunting hounds. Just as the dogs cannot be coaxed into following the trail, the teacher says he can no longer cajole the students to read and write. 

How long have they tugged the leash, and strained apart,
My pack of unruly hounds! I cannot start
Them again on a quarry of knowledge they hate to hunt,
I can haul them and urge them no more.

 
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