His book was written whilst he was Secretary to the Zoological Society of London, and made use of his remarkable collection of reprints covering the first part of the century.

Mather noted, too, that Huxley emphasises evolutionary progress, not by assuming that it means specialisation or "improvement" over earlier forms, but that attaining greater control over the environment and independence from it show man's progress at a new direction or rather at a higher level with "increases of aesthetic, intellectual, and spiritual experience and satisfaction.

Lamm calls it remarkable that both books were described as popular accounts at the time, and notes that Huxley states in his introduction that he was setting out to promote a "synthetic point of view" on Darwinian evolution.

The 1942 text however was "a more tolerant, pluralistic, view of evolution than we have come to expect in the wake of the "hardening of the synthesis" (as Stephen Gould so aptly put it), with plenty of coverage of two "rebels", the discoverer of chromosomal crossover, Cyril Darlington, and the "avowedly anti-neoDarwinian" mutationist Richard Goldschmidt.

Lamm notes, too, that Huxley anticipated evolutionary developmental biology's attention to embryonic development for understanding evolution.