Conversation so far:
What is Zap relative to Robert Crumb?
Independent publisher Don Donahue invited Robert Crumb to make a comic book; Crumb drew up two issues of Zap Comix, and Donahue published the first in February 1968.
What is significant about Robert Crumb's Zap Comix?
Robert Crumb had difficulty at first finding retailers who would stock it, and at first his wife took to selling the first run herself out of a baby carriage.
Were Zap Comix by Robert Crumb successful?
Zap Comix by Robert Crumb was financially successful, and developed a market for underground comix.

Question: What happened after this?
Self-contained rewrite: What happened after Zap Comix by Robert Crumb was financially successful?