200,000,000 guinea pigs, of course. Well, unless you are the guinea pig, which is what these books seem to be implying.
What's even more interesting? We have four (yes, I counted, four!) editions of 100,000,000 guinea pigs: 1932,1933,1935, and 1937. AND we have four (again, yes, four) copies of 200,000,000 guinea pigs. All I can envision are the poor expository writing instructors in the 1930's reading argument paper after informative paper after investigative paper about the toxins and dangers of hair dyes and shortening.
Thanks to Lydia and Brent for finding these gems! And, Lydia? You can stop giggling now. Honestly, any time now.