William is primarily notable as a subject of Charles Darwin's studies of infant psychology. Darwin was very fond of his son; at his birth he called him "a prodigy of beauty & intellect", and named him after his own grandfather Erasmus Darwin. During William's first three years his father kept a diary of gestures and facial expressions the infant made. The studies were part of Darwin's comparison between animal and human development, after he had already thoroughly studied orangutan babies at the London Zoo. The diary contains observations on the child learning to follow a candle with his eyes after nine days, smiling with his eyes after six weeks and three days, and developing distinctive cries adjusted to specific situations after eleven weeks. He also noticed the development of more profound personality traits, such as reason and, at two-and-a-half years, conscience.
