Perhaps it is a poem about finding one's true self. The poet watches a young snake and feels a connection and a longing. He recognizes something in the snake that he wants to nuture in himself:
I longed to be that thing.
The pure, sensuous form.
He watches the young snake "glide" out of a patch of "mottled shade". The shade could represent the safety or security of the place he habitually stays. For a snake to come into the sun is to risk exposure. Although still in nature, it is in a part of nature that leaves him vulnerable. However, the snake is not afraid--he is said to "hang, limp on a stone" with his tongue "stayed." When he turns to leave, he does so quickly, but without panic. It seems to leave as easily as it has appeared.
The poet sees the snake as an embodiment of pure sensuality, and he reacts to what he sees in a physical way. His blood "warms" and he realizes that he wants to be like the snake. At the same time, he realizes that he has a long way to go to get there.
In this poem, the persona recognizes that there is some disparity between who he is and who he wants to become. What he wants is the easy and pure form of sensuality that he sees the snake has. It is not sexual or forced. It is in its very being.
The physical response indicates that there is already a connection between the two, that the poet already identifies with the snake. His wistful comment at the end suggests that it is something that, in time, he has the potential to achieve.
Of his childhood in Saginaw, Michigan, where his German grandfather and father kept greenhouses, Theodore Roethke wrote that this greenhouse world represented for him "both heaven and hell, a kind of tropics created in the savage climate of Michigan, where austere German-Americans turned their love or order and their terrifying efficiency into something truly beautiful."
In his poem "Snake," Roethke does just this: He turns something terrifying into something beautiful. As one of his nature poems, "Snake" explores some of the anxiety that was with Roethke since his childhood. But, the power of Nature to revive the spirit of an adult life in self-realization is the theme of this poem.
The sight of the young snake gliding "Out of the mottled shade," which can be a metaphor for Roethke's anxieties, halts the poet, who watches the reptile. This snake is perceived as "a thin mouth and a tongue" that is "stayed" in the still air. But, after its limp pause upon a stone, it turns, draws away, and quickly is gone.
The sensuousness of the snake--the poet's "slow blood...longed to be that thing"--revives the man, giving him encouragement that he, too, can have relaxation and enjoy life: "And, I may be, some time." The poet realizes that he can experience release from his anxieties.