In Epperson v. Arkansas, 393 U.S. 97 (1968), the Supreme Court considered an Arkansas law that made it a crime "to teach the theory or doctrine that mankind ascended or descended from a lower order of animals," or "to adopt or use in any such institution a textbook that teaches" this theory in any school or university that received public funds. The court's opinion, written by Justice Abe Fortas, ruled that the Arkansas law violated "the constitutional prohibition of state laws respecting an establishment of religion or prohibiting the free exercise thereof. The overriding fact is that Arkansas' law selects from the body of knowledge a particular segment which it proscribes for the sole reason that it is deemed to conflict with a particular religious doctrine; that is, with a particular interpretation of the Book of Genesis by a particular religious group." The court held that the Establishment Clause prohibits the state from advancing any religion, and that "[T]he state has no legitimate interest in protecting any or all religions from views distasteful to them." 
Is there an answer to this question (If it cannot be answered, say "unanswerable"): What did the Arkansas law violate the constitutional prohibition of state laws in respect to?
an establishment of religion or prohibiting the free exercise thereof