7 April 1852
Went to the Zoo.
I said to Him—
Something about that Chimpanzee over there reminds me of you.
The date does not really signify any event. The date makes the poem seem like a diary entry. Also, ‘two’ rhymes with ‘zoo’ and ‘you’, which adds to the sing-song effect. 
  It is also important to note that Darwin’s theory of evolution was published in 1859, and the year chosen by Duffy predates that.  This suggests that Mrs. Darwin’s comment was the source of the theories that her husband developed.
Mrs Darwin