The Descent of Man

A very difficult read. I read it as an eBook on my iPhone, which made it doubly difficult because all the footnotes are interlineated in the text.
Historically, the book is very important. Darwin had left off dealing in detail with man's place in evolution in The Origin of Species because he knew it would be a lightning rod, and, as such, a distraction from his exposition of principles of natural selection.
I found the book's organization somewhat confusing. Only the first part of the book is really devoted to analyzing man's place in the natural world and how he has evolved. The second part of the book is devoted to sexual selection, surveying sexual characters throughout the animal kingdom. The third part attempts to tie together the first two parts, examining sexual characters in man.
Throughout this book, I was struck by how little data Darwin actually had on which to base his inquiries. One hundred forty years later we have exponentially more data on species, their genomes, relations, and behaviors. Hence, much of Darwin's discussion is speculative, as he readily acknowledges. DNA was unknown to Darwin, so he speculated about “gemmules” being transmitted to offspring. Much of his speculation about the relations among species is outdated by the precise information we have from DNA sequencing.
Unfortunately, Pasteur's insights into the crucial importance of microbial life, which were being formulated at that time, seem to have come a little late to have much influence on The Origin of Species and The Descent of Man. In reading this book, I can easily find flaws in his speculation on particular subjects: we now know about the importance of symbiosis as a mechanism driving evolution thanks to the work of Lynne Margulis; we know about the influence of geography, native species, natural resources, and infectious diseases in accounting for cultural and technological differences that Victorians might have attribute to race (e.g., see Jared Diamond's Guns, Germs, and Steel and Collapse; and the language about lower and higher species, savage and civilized races, now seems hopelessly Victorian.
Darwin was a trailblazer. This book reminds us how little Darwin could see from where he stood in the mid-19th century. We must also remember where he started and how far he came. His broad vision of the web of life and man's place in it was insightful and revolutionary.