BEHAVIOR--fresh, native, copious, each one for himself or herself,
Nature and the Soul expressed--America and freedom expressed--In it
the finest art,
In it pride, cleanliness, sympathy, to have their chance,
...
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To me it points out the differences between people, not depending on race, gender or all around looks, but by their behavior. Each person has their own behavior. And it's not only actions that determine the person but also they way the go about these actions.
the poem 'behavior' to me describes that the actions we choose to partake in can be the greatest determing factor of the remainder of our lives. bad choices lead down the wrong road, while the correct make your life easier and heading in the right direction to happiness.
Behavior is what defines a person in the world. Without expressing your purpose and character there is really no point to them. Walt Whitman is saying that therefore behavior is more 'effective' than anything else in the universe.
This poem comes across to me as a poem about a land of great diversities. Although people go about their daily activities regardless of who they are, they're behavior is what sets them apart from the rest.