In his remarkable paean of praise to the humble butterfly, Higginson starts off by describing it as "Thou spark of life that wavest wings of gold."
The comparison of the butterfly to a spark has two meanings here. First of all, it refers to the butterfly as somehow encapsulating the essence of life. And secondly, it draws our attention to the beautiful golden color of the butterfly's wings, a gorgeous hue that reminds the speaker of a bright spark.
If the butterfly represents the essence of life, then that essence is freedom. As well as being beautiful, the butterfly is also free. It wanders among “the songful birds” with nature's secrets contained in the color of its wings. Its irrevocable freedom stands in stark contrast to the flowers that remain "Still held within the garden's fostering."
Later on in the poem, the speaker compares the butterfly to other creatures—such as birds and field-mice—that lack the freedom of "Nature's freeman." Higginson was an ardent abolitionist, and some critics have argued that the entire poem is an allegory of the freedom he wished to see given to America's vast slave population.
Whether this is true or not, there can be no doubt that Higginson regards freedom as an essential element of the human condition and that he illustrates that freedom through the figure of the butterfly, a creature whose very essence is freedom, the spark of life.