Cunningham drew heavily from Walt Whitman’s "Specimen Days," using Whitman’s poetry and philosophy as a thematic and structural backbone for his novel. He was inspired by the idea that Whitman, despite living in a “hellish” 19th-century New York, found beauty in everything and shaped a sense of modern aesthetics.

The novel also reflects Cunningham’s engagement with existential fears stemming from historical and contemporary crises, such as the Cold War’s threat of annihilation and more recent technological and societal disruptions. These anxieties are woven into the narrative, particularly in the futuristic segment of the book, which explores themes of environmental degradation and technological collapse.

Humanity and Industrialization
The first story "In the Machine" is set during the Industrial Revolution and examines the dehumanizing effects of industrialization and the loss of individual identity. Lucas fears the machines are consuming human souls, symbolizing how technological progress can diminish human worth and connection. Whitman's transcendental poetry affirms the importance of the individual human spirit against the mechanization of society.

Alienation and Connection
Throughout the novel, characters struggle with feelings of alienation and seek meaningful human connection. In "The Children's Crusade", the child suicide bombers represent an extreme manifestation of alienation and disconnection from society. The futuristic "Like Beauty" depicts human/alien encounters that explore ideas of otherness and bridging vast divides.

Cycles of Violence and Hope
The novel examines cycles of violence, from the industrial "machine" deaths to the 9/11-inspired terrorism of "The Children's Crusade". Yet it also offers hope - the alien civilization of the future represents the possibility of overcoming violence and achieving harmony between disparate beings.

Whitman's Transcendentalism
Walt Whitman's poetry celebrating the beauty and divinity within each person is a unifying thread. His transcendental vision provides solace amid dehumanization and alienation, offering a path toward empathy, unity and spiritual rebirth across the narratives.

In grappling with industrialization, violence, alienation and human disconnection, "Specimen Days" uses Whitman's inclusive, democratic vision as a guide toward understanding our shared humanity across all boundaries.

The novel has generally received positive critical acclaim for its ambitious structure and thematic depth. Each of the three sections of the novel — a historical fiction piece set in the 19th century, a contemporary thriller, and a dystopian sci-fi story — are interconnected by recurring motifs, characters, and references to Walt Whitman’s poetry, particularly his work Leaves of Grass. The innovative structure has been praised for its creativity and for challenging conventional storytelling techniques.

Cunningham's ability to create compelling characters that resonate across different timelines has been positively reviewed. Despite the shifting settings and genres, the core emotions and struggles of the characters provide a continuous thread throughout the novel.

Cunningham's prose style has been consistently praised for its lyrical quality and emotional depth. His ability to capture the beauty and complexity of human experiences is a standout feature of the novel.

Reviews from major publications like The New York Times and The Guardian have generally been favorable, acknowledging the novel’s depth and Cunningham’s skillful execution of a complex narrative structure. Some critics, however, have noted that the ambitious structure can at times feel disjointed, and the shifts between genres may not appeal to all readers equally.

In summary, while "Specimen Days" may challenge readers with its ambitious narrative shifts and thematic complexity, it is widely regarded as a powerful and thought-provoking novel that successfully weaves together different genres and eras with poetic finesse.