"Will generation after generation / Turn with reverence to the page?"
These lines are the first indication the MacLeish is not literally referring to a snowflake in this poem, but rather, something that has been written. There seems to be a preoccupation with written work maintaining its value and popularity over many generations. The use of the phrase "with reverence" indicates that the speaker, and likely the author, is concerned with being admired and remembered.
"To be, yes!—whether they like it or not!"
In this line, MacLeish has taken the most famous part of Hamlet's speech and added an affirmation—leaving out the other possibility "not to be." Even further, he includes "whether they like it or not!" which implies that there should be no question about choosing existence, despite any protest from others about this choice.
"They also live / Who swerve and vanish in the river."
These lines serve to complicate the positions raised earlier in the poem, as the speaker is now acknowledging that even those beings that vanish also live. One could argue that whether recognized for generations to come, or vanishing from sight, all lives are equally fleeting, but also equally important. The syntax of the line "They also live..." evokes a tone of honor and respect for (and is perhaps a defense of) living things that may not always be acknowledged during or after their lives.
