In 1822 Eras went on to a medical course at Christ's College, Cambridge. When it came to be time for his one year external hospital study in 1825 he went to Edinburgh University, accompanying his younger brother who was just starting a course there in medicine. They planned ahead, Erasmus thinking "It will be very pleasant our being together, we shall be as cozy as possible", arriving early at Edinburgh to make social contact with old friends of the family in Whig society, and so "we can both read like horses". Erasmus enrolled with John Lizars, a "charming" and respectable surgeon on the other side of Surgeon's Square from his chief rival as a private tutor, the flamboyant Robert Knox who two years later became embroiled with the body-snatchers Burke and Hare. At the end of his hospital study year Erasmus enrolled in a London anatomy school, leaving Charles behind.
By 1828 Erasmus was ready to sit his Bachelor of Medicine exam at the University of Cambridge, and early in the new year he was accompanied to Cambridge by his brother Charles who had given up on medical studies and was now starting a course to qualify as a clergyman. That summer he went on a Continental tour to Munich, Milan and Vienna, and on his return home at the Christmas holiday he and Charles visited London, touring the scientific institutions.
There was an open secret in the family in 1833 that Erasmus was carrying on with Fanny Wedgwood, Hensleigh Wedgwood's wife, and as his sister Catherine wrote to Charles, "Papa continually prophesies a fine paragraph in the Paper about them". The Wedgwoods had a baby, ending his "junkitting at her house", but the affair resurfaced so Erasmus was "paired off" with Emma Wedgwood to avert "an action in the Papers."
In October 1836 after Charles returned from the voyage he stayed with his brother in a bustling London, where Erasmus enjoyed a life of literary leisure, his week revolving around intellectual dinner parties, spending his days "driving out Miss Martineau". Their father was concerned that her radicalism made her unsuitable as a daughter-in-law, and possibly a bad influence on his boys. Charles wrote that "Our only protection from so admirable a sister-in-law is in her working [Eras] too hard. He is beginning to perceive.. he shall be not much better than her 'nigger'. – imagine poor Eras a nigger to so philosophical & energetic a lady... She already takes him to task about his idleness." Charles called on Martineau and remarked that "She was very agreeable, and managed to talk on a most wonderful number of subjects, considering the limited time... I was astonished to find how ugly she is... she is overwhelmed with her own projects, he own thoughts and abilities", though "Erasmus palliated all this, by maintaining one ought not to look at her as a woman."
Eras had a cosmopolitan circle of friends including Martineau and his relative Hensleigh Wedgwood, and arranged intimate dinner parties with guests such as Charles Lyell, Charles Babbage and Thomas Carlyle. Radical and dissenting "heterodoxy was the norm". In the Spring of 1837 Charles moved to nearby lodgings where he could readily visit and attend Eras's dinner parties. In April 1838 Charles sent his parents the gossip that Miss Martineau had been "as frisky lately as the Rhinoceros. – Erasmus has been with her noon, morning & night: – if her character is not as secure, as a mountain in the polar regions she would certainly lose it".
Before marrying at the start of 1839 Charles moved to Gower Street, London, astounding Erasmus with the amount of his luggage. In the summer of that year Martineau's health broke down during a visit to the Continent and, fearing a tumour she retired to solitary lodgings in Tynemouth near her brother. She and Erasmus remained on good terms, writing to each other. Erasmus's social circle drifted apart, while "[Eras] sticks to his opium with many groans." (Opium was a common medicine at this time.)
As the reaction to Darwin's theory began following publication of The Origin of Species at the end of 1859, Erasmus thought it "the most interesting book I have ever read", and sent a copy to his old flame Harriet Martineau who at 58 was still reviewing from her home in the Lake District. In 1863 he was on the Council of the abolitionist Ethnological Society of London which at the time of the American Civil War was engaged in debate with the breakaway pro-slavery Anthropological Society.
Francis Galton, having caught the fad for Spiritualism, arranged a séance in January 1874 at Erasmus's house with those attending including Charles, Hensleigh Wedgwood and Thomas Huxley. Charles's son George hired the medium Charles Williams, and they sat round the table in the dark, though Charles left to lie down, missing the show. It "took away all their breaths" with a ringing bell, rushing wind, jumping candlestick and the table rising up above their heads. Galton thought it a "good séance" and Erasmus dabbled in "spirit photographs", but Charles remained convinced that it was "all imposture", as Huxley and George proved at a second séance.
By the autumn of 1880 Erasmus was in poorly health, suffering from the effects of time and opium, in constant pain and scarcely able to leave home. In September 1881 Charles stayed with Erasmus while his portrait was painted by John Collier. Shortly afterwards, Erasmus became gravely ill and died quietly on 26 August. Emma Darwin broke the news to Charles, who commented that he had seen Erasmus dying slowly "for many years", not "a happy man" but always kindhearted, clearheaded and affectionate. The funeral at Downe on 1 September was taken by their cousin John Allen Wedgwood, now 85, who had officiated at the wedding of Charles and Emma. Charles, looking "old and ill" in a long black funeral cloak, was a picture of "sad reverie" as the coffin was lowered. A marble slab was arranged, with words by Thomas Carlyle: "one of the sincerest, truest and most modest of men".