Finite

Episodic in nature, The Martian Chronicles features twenty-eight thinly connected vignettes that Ray Bradbury described as “a book of stories pretending to be a novel”. The Martians play very small roles, and as characters they are quite forgettable. Portrayed neither as heroes, nor villains, the aliens have a finite essentiality to the plot and when they supposedly die-out the story merely continues without looking back, which reinforces the book’s notion that no matter how technologically advanced a civilization, it is bound to fall. Mars is the only constant in this ever changing cast of characters.
For example, the marital woes of Ylla and Yll and the desperate attempts of an unnamed taxpayer to board a departing spaceship have very little to do with one another, but ultimately they do have common ground in that they all eventually perish.

Bradbury was influenced by Edgar Rice Burroughs’s Barsoom series and the similarity is quite evident; after all, “A Princess of Mars” is one of the first books to instill the idea of Mars as a habitable planet. There’s a certain familiarity we share with Mars; it is close in both size and relative proximity, and overall, makes the perfect replacement for earth.

The residents of Earth don’t occupy The Red Planet like invaders; just the introduction of a new species with simple biological differences. After the sporadic growth of trees in “The Green Morning”, the influx of colonists in “The Locusts”, and the eventual devastation of Earth life, and in what feels like a brief instant the Martians are history.

The last line in the book is the most telling. Surviving humans board a lone rocket for Mars, to start anew. With the last mementos of Earth destroyed, the refugees look into a pool and see–themselves. “The Martians were there—in the canal—reflected in the water…. The Martians stared back up at them for a long, long silent time from the rippling water….”
It is the perpetuation of the species, or the survival of the fittest; a type of evolutionary change, where the original Martians succumb to the stronger species of Earthlings, and that race continues until some other comes along.