But when it began advancing again, a real panic seized me, I stepped swiftly back, pointed my revolver point blank at the animal's huge chest and cried a warning to Oakley. As my forefinger closed over the trigger of the automatic, it seemed to require every scrap of my strength to exert the necessary pressure. There was a sharp double crack as Oakley and I fired almost simultaneously, then two more in quick succession from my own revolver, and a deep groan which had something terribly human in it.

The gorilla had fallen face forward on the grass, where it was twitching convulsively, and a minute later it was dead.

If ever a man felt like a murderer I did.

"By my godfather's . . ." breathed Oakley, "that was a thundering near thing. Why didn't you fire before?"

"Why didn't you?" I gasped.

"I couldn't!"

"Neither could I. I had to be driven to it by sheer desperation and danger. I don't like the look of the brute; it's too human. Let's get away again, before its brothers and cousins arrive. We're out here to catch 'em, not kill 'em, and I don't want any more blood on my hands to-day."

Oakley gave a short, high-strung laugh, and without further delay crossed over to the machine.

After a quarter of an hour's examination he made some startling discovery which he tried to explain to me in the usual technical jargon.

"What's the cause?" I asked, trying to sound intelligent.