Ace Double D-375 (1959) - Masters of Evolution by Damon Knight, bound with Fire in the Heavens by George O. Smith. This is the first paperback (or only) appearance for either novel.

Masters of Evolution, by Damon Knight - cover by Ed Emshwiller; 96 pages. Expanded (?) from "Natural State" - Galaxy January 1954. As far as I know, this book verson has never been reprinted.

SHOWDOWN WITH MONSTERS!
Alvah Gustad was typical of New York City's twenty million citizens. He took for granted such luxuries as synthetic food and robot-servants, and he knew beyond a doubt that the Cities offered the only acceptable way of life for civilized Man!

But roaming the vast plains between the continent's five Cities, ever growing and expanding, were the dreaded tribes of Muckfeet. In direct antithesis to the City dwellers, these illiterate savages actually grew food and raised animals. And how they smelled! It was so bad that Alvah could feel his stomach churn at he mere mention of their name.

There was one thing in the Muckfoot territory that the Cities did need, though - metal ores. And Alvah, faced with the job of liason to the tribes around New York, had to somehow make his patriotism outweigh his nausea. If he succeeded, the Cities would be monuments to eternity; if he failed, the dreaded Muckfeet would become the MASTERS OF EVOLUTION.

(Bound with) Fire in the Heavens George O. Smith - cover by Ed Valigursky (no signature - but no one else draws ocean-liners in space like he does); 159 pages. The hardcover of this came out from Avalon in 1958. A magazine version was serialized in Startling Stories July 1949 - and I believe this is slightly expanded from that.

OPERATION WORLD'S END
Jeff Benson and Lucille Roman stood worlds apart in their approach to science. Jeff, and instrument engineer, was interested only in its purest aspects. Lucille, a rocket manufacturer, thought only of its profit-making potential.

It was a one-in-a-million chance that their worlds would ever collide. But when the sun threatened to go nova - to incinerate the Earth in one devastating explosion - these two found themselves in a highly combustible partnership.

Only Jeff could possibly avert the coming catastrophe, and he could only do it with Lucille's co-operation. Upon the success of their emotion-charged alliance hung more than the future of science. The very fate of Earth depended on their working - or dying - together!