In November of that year he was nominated by the Board of Natural Science Studies to work at the Zoological Station at Naples, then lately established by Dr Anton Dohrn. His object in going there was to continue his investigations on Development, and before starting he had determined to study the Elasmobranch Fishes (Sharks and Rays), as it seemed likely, from their pristine characters, that their development would throw great light on the early history of vertebrate animals. The result showed how wisely he had made his selection. He made discoveries of the highest value in reference to the development of certain organs, and the origin of the nerves from the spinal cord—points which had baffled the most acute previous observers. These were not merely valuable for the history of the special group from which they were derived, but threw a flood of light upon the connexion between vertebrates and invertebrates, and their derivation from a common ancestry; views which he expanded afterwards in his work on Embryology. The results of his Neapolitan researches were embodied in the dissertation upon which he rested his candidature for a Fellowship at Trinity College; and were afterwards printed in the _Philosophical Transactions_ for 1875. Fortunately for him, a Natural Science Fellowship was vacant in 1874, to which he was elected, in consequence of the value of this dissertation. It is what is called an open secret that its great merits were at once recognized by Professor Huxley, to whom it had been referred.
From that time forward Balfour devoted himself unremittingly to continuous research in preparation for his systematic treatise on Embryology, the plan of which he had already sketched out, and which was finally completed and published in 1881. Before this appeared, however, he had published numerous papers of great value, covering nearly the whole range of his subject. Many of these will be found in the _Quarterly Journal of Microscopical Science_, of which he was one of the editors. As an original investigator he had no equal. He was skilful in manipulation, and observed rapidly and exactly, so that no point escaped his notice. His mind was calm and wholly free from prejudice, with a singularly broad and original grasp, which enabled him to seize, with readiness and sureness, the principle which lay under a number of apparently discordant facts. At the same time, like every true genius, he was singularly modest and retiring, always ready to depreciate the value of his own work, and to put forward that of others, especially of men younger than himself. We know of many students, now rising to distinction, who owe their first success to his generous encouragement, and, we may add, in some cases to his bountiful assistance, given with a delicacy which doubled the value of the gift. It was this strong desire to encourage others to work at Natural Science that induced him, in 1875, to undertake a class in Animal Morphology, or, as it used to be called, Comparative Anatomy. At first only a few students presented themselves, and one small room at the New Museums was sufficient for their accommodation. The class, however, grew with surprising rapidity; and, after Mr Balfour’s appointment as Natural Science Lecturer to Trinity College, it became necessary to build new rooms for his use. During the year 1881 the numbers had reached an average of nearly sixty in each term; and just before he left England for the excursion which has ended so fatally he had superintended the plans for a yet further extension of the Museum Buildings.