In 1786 he had communicated to Mr. Hunter, in the form of an essay, the result of several years’ careful observation of the singular habits of the cuckoo, till then a mystery to naturalists. It was presented by Mr. Hunter to the Royal Society, and was printed entire in their Transactions in 1789, having been returned to Jenner in the mean time, in order that he might record some additional facts which he had ascertained. This tract has been considered as a very masterly performance, and was the occasion of the author being elected to the fellowship of the Royal Society. It is not a little remarkable that Mr. Hunter, like Jenner’s friends at Alveston, thought so doubtfully of his views on the subject of vaccination, that he cautioned him against publishing them, lest they should interfere with the fame he had acquired in the learned world by his ‘Essay on the Cuckoo.’ But the event proved that the caution, though well meant, was unnecessary. Jenner was not more disposed than his gifted master to admit any conclusion on merely collateral grounds, that might be put to the test of experiment. This, however, was too new and important a matter to be lightly or prematurely hazarded; and Jenner waited long and patiently for an opportunity of thus testing his opinions, losing in the mean time no occasion of collecting additional information. The idea, thus watchfully and laboriously improved, was first excited in his mind while he was an apprentice at Sodbury, by a remark accidentally dropped by a young countrywoman in his master’s surgery, who, overhearing a conversation about the small-pox, observed that she had no fear of catching that disease as she had taken the _cow-pox_. Jenner, who was always alive to any subject connected with natural history, was induced to make more particular inquiries into this complaint, of which he had never heard before; and the answers he received were such as to suggest the probability of substituting it with advantage for the inoculated small-pox. Of this theory he never lost sight till he established it on the clearest evidence, and with it his unrivalled claim to the perpetual gratitude of mankind.