In The Expression of the Emotions in Man and Animals Charles Darwin argued that emotions put forward by facial expressions were the products of natural selection. He compared modern facial expressions in various people and found the same basic movements regardless of the persons ethnic or cultural background. He also compared human and animal expression and found many startling similarities. He thought facial expression was not a learned behavior but somehow innate. This condition could only be explained by common descent, which was a rather radical idea for the time.