In the poem "If," what does the speaker declare will be "yours" in line 31?

Line 31 of “If” reads thus:  “Yours is the Earth and everything that’s in it.”  The speaker is declaring to his son that if he can succeed in being and doing all the things outlined previously in the poem, he can achieve anything in the world.  And what this means, more importantly that being able to achieve whatever he sets his mind to, if he behaves nobly and calmly and studiously, if he lives a dedicated life and not a lazy one, then he will “be a man;” according to the speaker, all the conditions outlined in the poem are necessary for one to live a good life, and being a man is dependent on all of them.

This is a poem full of advice on how to be successful in life; in each verse Kipling adds condition after condition, building up until the final few lines, in which we see the result.  And thus the reader understands that achieving the world is based on our own behaviors and our own attitudes, rather than on anyone else.  It is on one’s own shoulders, and while the execution of these ideas may be difficult at times, the ideas themselves are really quite simple.