The book's somewhat controversial[1] argument is that evolution in animals (though no doubt similar processes occur in other organisms) proceeds mostly by modifying the way that regulatory genes, which do not code for structural proteins (such as enzymes), control embryonic development.

[3] He studies the evolution of cis-regulatory elements (pieces of non-coding DNA) which help to regulate gene expression in developing embryos, using the fruit fly Drosophila as the model organism.

The book's title quotes from the last sentence of Charles Darwin's 1859 The Origin of Species, in which he described the evolution of all living organisms from a common ancestor: "endless forms most beautiful and most wonderful have been, and are being, evolved.

[9] The book is illustrated with photographs, such as of developing fruit fly embryos dyed to show the effects of toolkit genes, and with line drawings by Jamie W. Carroll, Josh P. Klaiss and Leanne M. Olds.

He summarized the message of the book with the words "As Darwin's theory made clear, these multitudinous forms developed as a result of small changes in offspring and natural selection of those that were better adapted to their environment.

The review praised Carroll for tackling human evolution and covering the key concepts of what Charles Darwin called the grandeur of [the evolutionary view of] life, suggesting that "Kipling would be riveted.

"[13] The journalist Dick Pountain, writing in PC Pro magazine, argued that Endless Forms Most Beautiful was the most important popular science book since Richard Dawkins's The Blind Watchmaker, "and in effect a sequel [to it].

He answered his own question about the "astonishing morphological diversity" of animals coming from "such a limited number of genes", praising Carroll's "insightful and enthusiastic" style, writing in a "witty and engaging" way, pulling the reader into the complexities of Hox and PAX-6, as well as celebrating the Cambrian explosion of life forms and much else.