When I started reading this I was dubious about the significance of the shared date of birth of Darwin and Lincoln, but the author soon convinced me that the coincidence was much deeper. Both men, in very different circumstances, carried out a revolution in thought that brought in the modern world. Both were respectable citizens, not wild eccentrics. They were family men to an extent that was rare at the time, and each lost a beloved child. They shared a belief in our common humanity and a loathing for slavery. Both believed in the power of evidence and argument to change minds. Each in his different way was a master of persuasive language.
Lincoln was a lawyer and saw that respect for the law and the Constitution must be the basis for a just society. He crafted his speeches in such a way that a complex logical argument led up to a memorable punch line in the plainest English. He was a reluctant warrior who suffered deeply from the death on both sides, and his own death prevented him from following through from the emancipation of the slaves to their enfranchisement.
Darwin was a biologist with both an extraordinary eye for detail and a clear vision of life as a whole. He also had the talents of a novelist, which enabled him to present the theory of natural selection in such a way as to make it almost irresistible, and he disarmed critics by anticipating their objections. It took him 21 years to go from conception to publication, partly because he did not want to upset the religious convictions of his wife, whom he loved dearly.
Gopnik is himself a fine narrator and stylist, and he says as much in this one short book as others have taken volumes to convey.