The 1940s were a pupil-poor and scientifically unsettled time for U.S. secondary school biology textbook authors and publishers. The economic depression of the preceding decade led directly to a reduction in both birth rates and absolute births. Between 1940 and 1947, the number of students eligible for high school biology decreased by more than 13%.  Making matters worse, World War II interrupted the free flow of scientific information right as new ideas about evolution, eugenics and race were coming into focus, forcing (or encouraging) authors to rely on old information or kind of wander around.
This created an odd market where idiosyncratic works by authors on soapboxes competed for a share of a shrinking audience with barely revised reprints of out-of-date texts and encyclopedic works that purportedly instructed students how to “adjust” to life. Only a few of these weird creatures would survive the decade.
Of the 16 new or revised high school biology textbooks published during the 1940s, only two managed to successfully navigate the harsh environment. The first, the perennially popular (and pandering) Modern Biology by Truman J. Moon, Paul B. Mann and James H. Otto, first published in 1921, gained a strong foothold in the market with revised editions in 1926, 1933 and 1938. It’s unlikely rival, Exploring Biology, started life as the kitchen table project of a small-town high school teacher named Ella Thea Smith and her illustrator husband, Marion Cox. By the end of the 1950s, these two textbooks would command 75% of the market.