E.C. Tubb 
Enemy Within the Skull 
Cap Kennedy Book 4

FREE ACTING TERRAN ENVOYF.A.T.E.Alien scientists had learned how to release the ravening beast-instincts buried in the recesses of the human brain from the far distant past.Now they were willing to sell their knowledge to the highest bidder in exchange for an alliance against Terra. And they had many willing to bargain with them.Captain Kennedy, the top man of the Free Acting Terran Envoys — FATE — was assigned to stop this terrifying menace. But he knew immediately that his greatest danger lay within himself — and that by challenging the alien scientists he could turn himself and his friends into beasts beyond imagining!