"Crossing the Bar" is a poem by Alfred, Lord Tennyson. He wrote the poem during an illness and while at sea; he claimed that the words "came in a moment," and asked his son to place the poem at the end of each collection of his work (Wikipedia). The poem is often associated with death and dying, with the finality of age, and the poet's desire to move beyond "the bar" that separates life and death to meet "The Pilot" (God) at last. It is also a plea to his family not to mourn for him, because his life has been complete, and he feels that he has lived it to the fullest of his ability. This can be seen in the third stanza:
Twilight and evening bell,
And after that the dark!
And may there be no sadness of farewell,
When I embark;
(Tennyson, "Crossing the Bar," bartleby.com)
The poem acts as a final work, a sort of epitaph. With the poem at the end of a collection, the reader receives the idea that the work of the poet has come to a specific conclusion, and that there is nothing more to see here. As a reflection on life and death, it falls naturally after all other poems which concern themselves with events, people, and ideas; once the collection (life) is over, there is little more to say. It is not, however, a poem of sadness and misery; the reader understands that the poet has come to his own acceptable ending, and the reader can now move on to other collections (lives) and feel joy that the poet has lived and died in the manner he most desired.